Allow use of custom DataLoaderRegistry instance

[rstoyanchev]

Currently DefaultExecutionGraphQlService sets the DataLoaderRegistry in ExecutionInput to a new instance because it needs to make registrations, while the default instance in ExecutionInput is a no-op that raises exceptions.

We could improve this by using the DataLoaderRegistry from ExecutionInput provided that it's not the default no-op implementation, or has any registrations, which implies it's not the no-op. This would allow use of alternative DataLoaderRegistry like ScheduledDataLoaderRegistry.