[Gradle] Add withSourcesJar to gradle dsl

* Each target can be configured to either publish or not its sources.
* For convenience withSourcesJar available also on kotlin multiplatform
  extension level. It then calls `withSourcesJar` for each target.

^KT-55881 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinTarget.kt

@@ -38,6 +38,8 @@ interface KotlinTarget : Named, HasAttributes {
 
     val publishable: Boolean
 
+    fun withSourcesJar(publish: Boolean = true)
+
     val components: Set<SoftwareComponent>
 
     fun mavenPublication(action: MavenPublication.() -> Unit) = mavenPublication(Action { action(it) })