Allow configuring the set of workers to proxy outbound federation traffic through (`outbound_federation_restricted_to`).

This is useful when you have a worker setup with `federation_sender` instances responsible for sending outbound federation requests and want to make sure *all* outbound federation traffic goes through those instances. Before this change, the generic workers would still contact federation themselves for things like profile lookups, backfill, etc. This PR allows you to set more strict access controls/firewall for all workers and only allow the `federation_sender`'s to contact the outside world.

The original code is from @erikjohnston's branches which I've gotten in-shape to merge.