[gql_http_link] Consider adding more context options

[micimize]

• should/can other links add headers?
• should more http config options (like method) just be context? That way, useGETForQueries could eventually be a standalone a context update operation

For reference, I think the entire graphql/client.dart HttpConfig can be overridden through context, which is roughly:

class HttpQueryOptions {
  bool includeQuery;
  bool includeExtensions;
}


class HttpConfig {
  HttpQueryOptions http;
  Map<String, dynamic> options;
  Map<String, dynamic> credentials;
  Map<String, String> headers;
}