Client: PDS discovery?

[snarfed]

Now that Bluesky is internally federating, I'm thinking about how to handle that. Right now, Client either takes a single explicit server or defaults to bsky.social. Should we also support per-call server discovery based on the target repo's DID doc? Seems useful, but we'd have to determine the target repo for each call, which there's no call-independent way to do afaik, and some calls are PDS-level (not repo-level) anyway. Hrm.