Add spring-boot sample application

[ramsrib]

Context

Added spring-boot sample which is missing in the present kotlin-dsl samples.

Contributor Checklist

• [x] Base the PR against the develop branch
• [x] Make sure that all commits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
• [x] Provide integration tests to verify changes from a user perspective
• [x] Provide unit tests to verify logic
• [x] Ensure that tests pass locally: ./gradlew check --parallel

[ramsrib]

Build is failing in JDK 8. Any idea?

org.gradle.kotlin.dsl.integration.KotlinBuildScriptModelIntegrationTest > can fetch buildSrc classpath in face of compilation errors STANDARD_ERROR
    e: /tmp/junit638243372634683066/can#20fetch#20buildSrc#20classpath#20in#20face#20of#20compilation#20errors/build.gradle.kts:2:20: Expecting an expression
    org.gradle.internal.exceptions.LocationAwareException: Build file '/tmp/junit638243372634683066/can#20fetch#20buildSrc#20classpath#20in#20face#20of#20compilation#20errors/build.gradle.kts' line: 2
    Script compilation error:
      Line 2:             val p =
                                 ^ Expecting an expression
    1 error
    	at org.gradle.kotlin.dsl.provider.KotlinBuildScriptCompiler.withKotlinCompiler(KotlinBuildScriptCompiler.kt:508)

[JLLeitschuh]

@ramsrib Is that a test failure?

[ramsrib]

It doesn't look like a test failure.

[ramsrib]

Guys, Any idea on why the build is failing?