Add `application/graphql-request+json` media type?

[martinbonnin]

Follow up from #215 (comment)
See also #249

GraphQL servers have used application/json for the requests, basically forever.

If a server supports several query languages, it cannot easily disambiguate.

This issue is to gather interest for a specific application/graphql-request+json media type for GraphQL requests:

QUERY /graphql HTTP/1.1
Content-Type: application/graphql-request+json

{
  "query": "query GetUser($id: ID!) { user(id: $id) { name } }",
  "variables": { "id": "42" },
  "operationName": "GetUser"
}

If you have such a use case, please comment bellow