Feature Request - Metadata in AppSync Response

Please also add the ability to add metadata to the AppSync response. This is currently available as GraphQL extensions:

1) https://github.com/graphql-dotnet/graphql-dotnet/issues/912
2) https://github.com/graphql/express-graphql#providing-extensions

Start by reviewing the two linked references on GraphQL extensions and compare their metadata behavior with the current AppSync response. The issue does not name repository files, entry points, or tests, so first identify where AppSync response construction is documented or implemented. Done should include a defined metadata shape and an agreed response behavior, followed by coverage for that behavior.

Written by the indexing model from the issue text.