[Gradle] Initial 'externalTargetApi' w/ compilable Android prototype

^KT-54766 Verification Pending

libraries/tools/kotlin-gradle-plugin-annotations/src/main/kotlin/org/jetbrains/kotlin/gradle/ExternalKotlinTargetApi.kt

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2010-2022 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
+
+@RequiresOptIn(
+    message = "This API is intended to be used by Google to maintain KotlinTargets outside of kotlin.git",
+    level = RequiresOptIn.Level.ERROR
+)
+annotation class ExternalKotlinTargetApi