Add GraphQL::Query::Partial

[rmosolgo]

This is to implement partial execution, where you can take a prepared GraphQL::Query and run part of it in isolation.

Fixes #5136

TODO

• Finish unit tests
• Add specific tests for Dataloader and lazy resolution
• DRY between Query and Query::Partial
• What about when selections are duplicated in the AST? Have to merge fields appropriately
• Add tests for continuing from non-null fields, unions, interfaces, scalars, enums

[rmosolgo]

These were leftover from a previous GraphQL::Query::Context implementation. I rediscovered them while working on this feature because at first, I thought I was going to need to use query.context.dup. I didn't end up doing that (instead, Query::Context.new inside Partial#initialize) but I'm going to keep these clean-ups.