Use GraphQL::Query as request basis

[gmac]

This eliminates some redundancies between the stitching Request object and GraphQL::Query, and generally makes the stitching request behave more like a normal query. This includes:

• Adds variable validations; these were missing (breaking behavior, though fundamentally a bug).
• Non-null variables with only default values are caught by validations (matches base library).
• Removes Request.prepare! without replacement. It's now incorporated directly into request plumbing.
• Evaluates runtime possible types through the request query rather than the static schema (prepares for visibility).
• Operation error messages match the base GraphQL Ruby library.