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

Follow up from https://github.com/graphql/graphql-over-http/pull/215#issuecomment-5044418411
See also https://github.com/graphql/graphql-over-http/issues/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

Read the linked pull request 215 and issue 249 first, then review the proposed application/graphql-request+json example and the stated multi-query-language use case. Done means reaching a decision on whether this media type belongs in the GraphQL over HTTP specification, with the issue documenting the outcome.

Written by the indexing model from the issue text.