Implement exit support for relays
We want relays to have support for delivering traffic.
This will require support for exit policies, and DNS caching. We should determine what our actual best practices for DNS caching are: many of the things we have tried in the past are vulnerable to attacks. (See tor#40942.) We may also want to implement deduplication for concurrent DNS requests for the same resource, like C tor has.
This ticket will almost surely be broken into sub-tickets after initial design is done.