Add and enable new plugin extension configuration in Gradle

[chanseokoh]

(Gradle counterpart of #2420.)

Adds new extension configuration parameters for Gradle. Extensions can additionally get custom properties. The order of executing extensions will be the order defined in pom.xml.

The following example configures two extensions.

  pluginExtensions { pluginExtension { implementation = 'com.example.Test' } }
  pluginExtensions {
    pluginExtension {
      implementation = 'com.google.cloud.tools.jib.plugins.extension.gradle.quarkus.QuarkusJibExtension'
      properties = [customProperty: 'value']
    }
  }

That is, multiple pluginExtensions blocks are needed to configure multiple extensions. I've consulted the implementation of the Gradle plugin for Visual Studio Code IDE. I am not totally sure if this is the standard way, but I could not figure out a way to enable configuring extensions as a list like pluginExtensions = [ { ... }, { ... } ]. (UPDATE: now new syntax with latest commits.)

Originally the name of the block was extensions { ... }. Although it did work, one of our test classes complained that their's ambiguity in setExtensions(), so I went with pluginExtensions { ... }. Seems like an acceptable renaming. Probably I'll have to rename the Maven configuration to be consistent.

[loosebazooka]

The plural on the base jib config of pluginExtensions seems weird. I think either

jib {
  pluginExtension {..}
  pluginExtension {..}
}

or (and this matches most models I see like licenses and developers in pom config)

jib {
  pluginExtensions {
    pluginExtension {...}
  }
}

[loosebazooka]

I think this is good, but have a few comments

[chanseokoh]

jib {
  pluginExtensions {
    pluginExtension {...}
  }
}

Trying to find a way to make this work. I'll look into this later.

UPDATE: done. The name of the new intermediate class ExtensionParametersSpec followed the convention of MavenPomLicenseSpec.