logstash Set Imap plugin as a non-default plugin.

Release notes

[rn:skip]

What does this PR do?

Sets the imap plugin as a non-default plugin, so that Logstash will not carry it with its distribution.

Why is it important/What is the impact to the user?

We will try our best to keep the plugin up-to date.

Checklist

~~[ ] My code follows the style guidelines of this project~~
~~[ ] I have commented my code, particularly in hard-to-understand areas~~
~~[ ] I have made corresponding changes to the documentation~~
~~[ ] I have made corresponding change to the default configuration files (and/or docker env variables)~~
~~[ ] I have added tests that prove my fix is effective or that my feature works~~

Author's Checklist

[ ]

How to test this PR locally

Pull the changes
./gradlew clean bootstrap assemble installDefaultGems will install the default plugins where imap plugin will not be under vendor

Related issues

Use cases

Screenshots

Logs

When running Logstash without installing imap plugin:

➜  logstash git:(unbundle-input-imap-plugin) ✗ ./bin/logstash -f config/imap.conf 
Using system java: /usr/bin/java
Sending Logstash logs to /Users/mashhur/Dev/elastic/logstash/logs which is now configured via log4j2.properties
[2023-10-10T12:57:56,323][INFO ][logstash.runner          ] Log4j configuration path used is: /Users/mashhur/Dev/elastic/logstash/config/log4j2.properties
[2023-10-10T12:57:56,328][INFO ][logstash.runner          ] Starting Logstash {"logstash.version"=>"8.12.0", "jruby.version"=>"jruby 9.4.2.0 (3.1.0) 2023-03-08 90d2913fda OpenJDK 64-Bit Server VM 17.0.4.1+9-LTS on 17.0.4.1+9-LTS +indy +jit [arm64-darwin]"}
[2023-10-10T12:57:56,329][INFO ][logstash.runner          ] JVM bootstrap flags: [-Xms1g, -Xmx1g, -Djava.awt.headless=true, -Dfile.encoding=UTF-8, -Djruby.compile.invokedynamic=true, -XX:+HeapDumpOnOutOfMemoryError, -Djava.security.egd=file:/dev/urandom, -Dlog4j2.isThreadContextMapInheritable=true, -Djruby.regexp.interruptible=true, -Djdk.io.File.enableADS=true, --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED, --add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED, --add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED, --add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED, --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED, --add-opens=java.base/java.security=ALL-UNNAMED, --add-opens=java.base/java.io=ALL-UNNAMED, --add-opens=java.base/java.nio.channels=ALL-UNNAMED, --add-opens=java.base/sun.nio.ch=ALL-UNNAMED, --add-opens=java.management/sun.management=ALL-UNNAMED]
[2023-10-10T12:57:56,344][WARN ][logstash.config.source.multilocal] Ignoring the 'pipelines.yml' file because modules or command line options are specified
[2023-10-10T12:57:56,553][INFO ][logstash.agent           ] Successfully started Logstash API endpoint {:port=>9600, :ssl_enabled=>false}
[2023-10-10T12:57:56,633][INFO ][org.reflections.Reflections] Reflections took 46 ms to scan 1 urls, producing 132 keys and 464 values
[2023-10-10T12:57:56,649][ERROR][logstash.plugins.registry] Unable to load plugin. {:type=>"input", :name=>"imap"}
[2023-10-10T12:57:56,652][ERROR][logstash.agent           ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:main, :exception=>"Java::JavaLang::IllegalStateException", :message=>"Unable to configure plugins: (PluginLoadingError) Couldn't find any input plugin named 'imap'. Are you sure this is correct? Trying to load the imap input plugin resulted in this error: Unable to load the requested plugin named imap of type input. The plugin is not installed.", :backtrace=>["org.logstash.config.ir.CompiledPipeline.<init>(CompiledPipeline.java:120)", "org.logstash.execution.AbstractPipelineExt.initialize(AbstractPipelineExt.java:186)", "org.logstash.execution.AbstractPipelineExt$INVOKER$i$initialize.call(AbstractPipelineExt$INVOKER$i$initialize.gen)", "org.jruby.internal.runtime.methods.JavaMethod$JavaMethodN.call(JavaMethod.java:847)", "org.jruby.ir.runtime.IRRuntimeHelpers.instanceSuper(IRRuntimeHelpers.java:1318)", "org.jruby.ir.instructions.InstanceSuperInstr.interpret(InstanceSuperInstr.java:139)", "org.jruby.ir.interpreter.InterpreterEngine.processCall(InterpreterEngine.java:367)", "org.jruby.ir.interpreter.StartupInterpreterEngine.interpret(StartupInterpreterEngine.java:66)", "org.jruby.internal.runtime.methods.MixedModeIRMethod.INTERPRET_METHOD(MixedModeIRMethod.java:128)", "org.jruby.internal.runtime.methods.MixedModeIRMethod.call(MixedModeIRMethod.java:115)", "org.jruby.runtime.callsite.CachingCallSite.cacheAndCall(CachingCallSite.java:452)", "org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:92)", "org.jruby.RubyClass.newInstance(RubyClass.java:931)", "org.jruby.RubyClass$INVOKER$i$newInstance.call(RubyClass$INVOKER$i$newInstance.gen)", "org.jruby.runtime.callsite.CachingCallSite.cacheAndCall(CachingCallSite.java:452)", "org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:92)", "org.jruby.ir.instructions.CallBase.interpret(CallBase.java:561)", "org.jruby.ir.interpreter.InterpreterEngine.processCall(InterpreterEngine.java:367)", "org.jruby.ir.interpreter.StartupInterpreterEngine.interpret(StartupInterpreterEngine.java:66)", "org.jruby.ir.interpreter.InterpreterEngine.interpret(InterpreterEngine.java:88)", "org.jruby.internal.runtime.methods.MixedModeIRMethod.INTERPRET_METHOD(MixedModeIRMethod.java:238)", "org.jruby.internal.runtime.methods.MixedModeIRMethod.call(MixedModeIRMethod.java:225)", "org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:228)", "org.jruby.runtime.callsite.CachingCallSite.cacheAndCall(CachingCallSite.java:516)", "org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:293)", "org.jruby.ir.interpreter.InterpreterEngine.processCall(InterpreterEngine.java:328)", "org.jruby.ir.interpreter.StartupInterpreterEngine.interpret(StartupInterpreterEngine.java:66)", "org.jruby.ir.interpreter.Interpreter.INTERPRET_BLOCK(Interpreter.java:116)", "org.jruby.runtime.MixedModeIRBlockBody.commonYieldPath(MixedModeIRBlockBody.java:136)", "org.jruby.runtime.IRBlockBody.call(IRBlockBody.java:66)", "org.jruby.runtime.IRBlockBody.call(IRBlockBody.java:58)", "org.jruby.runtime.Block.call(Block.java:143)", "org.jruby.RubyProc.call(RubyProc.java:352)", "org.jruby.internal.runtime.RubyRunnable.run(RubyRunnable.java:110)", "java.base/java.lang.Thread.run(Thread.java:833)"]}
[2023-10-10T12:57:56,658][INFO ][logstash.runner          ] Logstash shut down.
[2023-10-10T12:57:56,659][FATAL][org.logstash.Logstash    ] Logstash stopped processing because of an error: (SystemExit) exit
org.jruby.exceptions.SystemExit: (SystemExit) exit
	at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:795) ~[jruby.jar:?]
	at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:758) ~[jruby.jar:?]
	at Users.mashhur.Dev.elastic.logstash.lib.bootstrap.environment.<main>(/Users/mashhur/Dev/elastic/logstash/lib/bootstrap/environment.rb:90) ~[?:?]