[K/N] Introduce NativeRuntimeApi annotation

Marks the Kotlin/Native stdlib API that is related to runtime behavior.
Such API is unstable and won't become stable in the future.

As a part of efforts to stabilize Native stdlib #KT-55765.

kotlin-native/runtime/src/main/kotlin/kotlin/native/runtime/NativeRuntimeApi.kt

@@ -0,0 +1,38 @@
+/*
+ * 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 kotlin.native.runtime
+
+import kotlin.annotation.AnnotationTarget.*
+
+/**
+ * This annotation marks the Kotlin/Native standard library API that tweaks
+ * or otherwise accesses the Kotlin runtime behavior.
+ *
+ * The API marked with this annotation is considered unstable and is **not** intended to become stable in the future.
+ * Behavior of such an API may be changed or the API may be removed completely in any further release.
+ *
+ * Any usage of a declaration annotated with `@NativeRuntimeApi` must be accepted either by
+ * annotating that usage with the [OptIn] annotation, e.g. `@OptIn(NativeRuntimeApi::class)`,
+ * or by using the compiler argument `-opt-in=kotlin.native.runtime.NativeRuntimeApi`.
+ */
+@RequiresOptIn(level = RequiresOptIn.Level.ERROR)
+@Retention(AnnotationRetention.BINARY)
+@Target(
+        CLASS,
+        ANNOTATION_CLASS,
+        PROPERTY,
+        FIELD,
+        LOCAL_VARIABLE,
+        VALUE_PARAMETER,
+        CONSTRUCTOR,
+        FUNCTION,
+        PROPERTY_GETTER,
+        PROPERTY_SETTER,
+        TYPEALIAS
+)
+@MustBeDocumented
+@SinceKotlin("1.9")
+public annotation class NativeRuntimeApi