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/tools/kotlin-serialization/src/main/kotlin/org/jetbrains/kotlinx/serialization/gradle/SerializationSubplugin.kt

@@ -23,7 +23,6 @@ import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation
 import org.jetbrains.kotlin.gradle.plugin.KotlinGradleSubplugin
 import org.jetbrains.kotlin.gradle.plugin.SubpluginArtifact
 import org.jetbrains.kotlin.gradle.plugin.SubpluginOption
-import org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile
 
 class SerializationGradleSubplugin : Plugin<Project> {
     companion object {
@@ -51,7 +50,7 @@ class SerializationKotlinGradleSubplugin : KotlinGradleSubplugin<AbstractCompile
         javaCompile: AbstractCompile?,
         variantData: Any?,
         androidProjectHandler: Any?,
-        kotlinCompilation: KotlinCompilation?
+        kotlinCompilation: KotlinCompilation<*>?
     ): List<SubpluginOption> {
         return emptyList()
     }