Coerce input values in a single pass

[rmosolgo]

For a long time, GraphQL-Ruby has validated before coercing -- but also implemented validation by coercing and checking for errors, discarding the coerced result.

It's time to fix this using the new input_values construct from Execution::Next. (Legacy execution will still coerce twice.)

Fixes #5535

TODO

• What about this JSON-with-nested variables test? It failed because the variable isn't defined. It used to also fail because of the value. Should I retain compatibility with previous?
• AstCoercionFailed is just a hack for now. Maybe I should move the error message up to coerce methods and remove the default overwrite in the validator and always used the message from the raised error
• Add a test that makes sure that coerce isn't called twice