Adds `a_graphql_entity_for`, a general matcher

This adds `GraphqlHelpers#a_graphql_entity_for`, which helps us write
expectations about results from GraphQL queries. In addition to the main
reason for this matcher (abstracting over `id: global_id_of(thing)`
checks), it handles simple field/method-name mappings and custom field
mappings, which accounts for 95% of all use cases.

This matcher can be converted back to a hash, which means it can be
destructured as `**a_graphql_entity_for(model)`.