Add API to configure Gradle extensions

[mhalbritter]

While looking at #1495 i noticed that there's currently no nice option to configure Gradle extensions.

This

kotlin {
	compilerOptions {
		jvmTarget = JvmTarget.JVM_1_8
		freeCompilerArgs += '-Xjsr305=strict'
	}
}

requires fiddling with GradleBuild.snippets() and manually constructing the coirrect syntax. It would be nice if GradleBuild could offer an API to to do that more nicely.

Maybe something like this?

build.extensions().customize("kotlin", (kotlin) -> kotlin.nested("compilerOptions", (compilerOptions) -> {
  compilerOptions.attributeWithType("jvmTarget", getJvmTarget(), "org.jetbrains.kotlin.gradle.dsl.JvmTarget");
  if (!CollectionUtils.isEmpty(this.settings.getCompilerArgs())) {
    compilerOptions.invoke("freeCompilerArgs.addAll", quote(this.settings.getCompilerArgs()));
  }
}));

[mhalbritter]

We also need to move Invocation and Attribute to top-level types and so some deprecation dance and inheritance.