[Serializer] Make metadata interfaces internal

src/Symfony/Component/Serializer/Mapping/AttributeMetadataInterface.php

@@ -16,6 +16,8 @@ namespace Symfony\Component\Serializer\Mapping;
  *
  * Primarily, the metadata stores serialization groups.
  *
+ * @internal
+ *
  * @author Kévin Dunglas <dunglas@gmail.com>
  */
 interface AttributeMetadataInterface

src/Symfony/Component/Serializer/Mapping/ClassMetadataInterface.php

@@ -18,6 +18,8 @@ namespace Symfony\Component\Serializer\Mapping;
  *
  * There may only exist one metadata for each attribute according to its name.
  *
+ * @internal
+ *
  * @author Kévin Dunglas <dunglas@gmail.com>
  */
 interface ClassMetadataInterface