[K2] Support legacy serialization for custom function type extensions.

For Compose libraries, it is important that libraries built with
K2 in version 2.0 can be read and used by a version 1.9 K1
compiler. That is not possible if custom function types are
in the metadata.

^KT-58456 Fixed

core/compiler.common/src/org/jetbrains/kotlin/builtins/functions/FunctionTypeKind.kt

@@ -63,6 +63,22 @@ abstract class FunctionTypeKind internal constructor(
     open val prefixForTypeRender: String?
         get() = null
 
+
+    /**
+     * Specifies the first language version for which to serialize custom function types
+     * into kotlin metadata. Until that version, custom function types will be serialized
+     * with the legacy scheme: as a FunctionN/KFunctionN with the annotation used for the
+     * custom function type.
+     *
+     * If no version is specified, custom function types are serialized to kotlin metadata.
+     *
+     * Serialization using the legacy format allows libraries compiled with K2 with a
+     * K2 plugin that uses custom function types to be used by clients using a K1 compiler
+     * with a K1 compiler plugin that understands the custom function types.
+     */
+    open val serializeAsFunctionWithAnnotationUntil: String?
+        get() = null
+
     /**
      * @return corresponding non-reflect kind for reflect kind
      * @return [this] if [isReflectType] is false