Add support for JSON Metadata to GraphQL Mesh

[mikepizzo]

GraphQL Mesh enables exposing an OData service through a GraphQL Gateway or Client SDK. It reads $metadata in order to generate a GraphQL schema and handlers for calling the OData service.

Unfortunately, GraphQL Mesh currently only works with csdl-xml.

In order to work well with RAPID services that only support csdl-json, we would need to enhance the GraphQL Mesh OData Handler to support csdl-json.

Note that OData Handler builds a local cached version of the metadata in a json format. Ideal would be if that format were (a subset of) csdl-json.

Note that once GraphQL Mesh supports csdl-json, we should update the GraphQL sample/tutorial to use csdl-json -- currently it forces application/xml.