[Gradle] Implement KotlinExtensionPoint to allow extensible designs inside the KotlinGradlePlugin

KT-61634

libraries/tools/kotlin-gradle-plugin-api/api/kotlin-gradle-plugin-api.api

@@ -896,6 +896,11 @@ public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinExecuti
 public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinExecution$ExecutionSource {
 }
 
+public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinExtensionPoint {
+	public abstract fun get (Lorg/gradle/api/Project;)Ljava/util/List;
+	public abstract fun register (Lorg/gradle/api/Project;Ljava/lang/Object;)V
+}
+
 public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinGradleSubplugin {
 }
 

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

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle.plugin
+
+import org.gradle.api.Project
+import org.jetbrains.kotlin.gradle.ExperimentalKotlinGradlePluginApi
+
+/**
+ * ### Extension Point for the Kotlin Gradle Plugin
+ *
+ * Note: Not stable for implementation: Custom implementations of this interface will not be supported by the Kotlin Gradle Plugin!
+ *
+ * #### Motivation
+ * This [KotlinExtensionPoint] will be used to 'generically' extend parts of the Kotlin Gradle Plugin (internally and externally).
+ * General Kotlin Gradle Plugin code will call into this 'semantically' structured extension points to load logic
+ * Such extension points are to be used for public as well as for internal extension points.
+ *
+ * ### Usage
+ * #### Declaring an Extension Point (internal in Kotlin Gradle Plugin)
+ * Extension Points are to be declared on companion objects on the corresponding interface
+ * ```kotlin
+ * interface MyKotlinGradlePluginExtension {
+ *     fun foo(project: Project)
+ *
+ *     companion object {
+ *         val extensionPoint = KotlinExtensionPoint<MyKotlinGradlePluginExtension>()
+ *     }
+ * }
+ * ```
+ *
+ * #### Registering an Extension Point
+ * Extension Points are scoped by [Project] and should be registered as early as possible
+ * ```kotlin
+ * class MyPlugin: Plugin<Project> {
+ *     fun apply(project: Project) {
+ *         MyKotlinGradlePluginExtension.extensionPoint.register(project, MyKotlinGradlePluginExtensionImpl())
+ *     }
+ * }
+ * ```
+ *
+ * #### Kotlin Gradle Plugin Entry Point:
+ * Extensions registered by the Kotlin Gradle Plugin directly will use the
+ * [org.jetbrains.kotlin.gradle.plugin.registerKotlinPluginExtensions] entry point.
+ */
+@ExperimentalKotlinGradlePluginApi
+interface KotlinExtensionPoint<T> {
+    /**
+     * @return all currently registered extension points.
+     * The returned list is *not* live and just represents the current snapshot.
+     */
+    operator fun get(project: Project): List<T>
+
+    /**
+     * @param project The current [project] to register an extension. The extension will only be visible to this particular [project]
+     * @param extension The implementation of the extension to register. If registered twice, it will be returned twice when
+     * the extensions are queried (no de-duplication)
+     */
+    fun register(project: Project, extension: T)
+}