Expose statically-typed kotlinOptions and compileKotlinTask in DSL

To simplify configuring a `KotlinCompilation` with Gradle Kotlin DSL, it
is essential to expose statically-known types where possible.

This commit parametrizes `KotlinCompilation` with its Kotlin options
type (a subtype of `KotlinCommonOptions`) and adds `kotlinOptions` and
`compileKotlinTask` to `KotlinCompilation`.

(minor) Also reduce the visibility of `attachClassesDir` and
`setupPlugins` to internal, since this API is not for external use.

libraries/examples/kotlin-gradle-subplugin-example/src/main/kotlin/example/ExampleSubplugin.kt

@@ -32,7 +32,7 @@ class ExampleSubplugin : KotlinGradleSubplugin<AbstractCompile> {
         javaCompile: AbstractCompile?,
         variantData: Any?,
         androidProjectHandler: Any?,
-        kotlinCompilation: KotlinCompilation?
+        kotlinCompilation: KotlinCompilation<*>?
     ): List<SubpluginOption> {
         println("ExampleSubplugin loaded")
         return listOf(SubpluginOption("exampleKey", "exampleValue"))