Add GraphQL::Current, test with ActiveRecord::QueryLogs

[rmosolgo]

I'm hoping to add tracing options using ActiveRecord::QueryLogs, also trying out Fiber::[], as suggested in #4993 (comment)

Fixes #4946

TODO:

• Consider adding Fiber[:current_multiplex] and sourcing operation_name from there
• Add unit tests for Current, test when no query is running
• Add to Rails generator