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
• Address TODOs in execution code
• Dry run_partials and run_partial_eager?
• 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
• Add some way to spawn a partial from a query, or halt execution in a query. ctx.skip?
  • context.skip works for now -- I can explore sugar for that down the road.
• Test lazy root object in Partial -- object and scalar
• Test base_path usage, current_path setting in Partial