Simplified Jib plugin extension configuration

[chanseokoh]

When using a single extension that doesn't define an extension-specific config, the user currently has to put quite some block into the build file.

• Maven

      <pluginExtensions>
        <pluginExtension>
          <implementation>com.example.Foo</implementation>
        </pluginExtension>
      </pluginExtensions>

• Gradle

  pluginExtensions {
    pluginExtension {
      implementation = 'com.example.Foo'
    }
  }

It'd be nice if they could do

      <pluginExtensions>com.example.Foo</pluginExtensions>

  pluginExtensions = 'com.example.Foo'

or the following if the above doesn't make sense:

      <pluginExtensions>
        <!-- element names in a list doesn't matter in Maven -->
        <extension>com.example.Foo</extension>
        <anotherExtension>com.example.Bar</anotherExtension>
      </pluginExtensions>

  pluginExtensions = ['com.example.Foo']

[saptaswapal]

Hi, I am new to this space and would like to take up this task and contribute to this project

[chanseokoh]

@Sap1996 thanks! Please have a look at #3214.