let ATP_PLC_HOST override default plc host

[lun-4]

I've been working on spinning up my own bluesky stack (including the did:plc reference server) and found out that only some commands on goat support a -plc-directory setting, but most don't because they just use DefaultDirectory() instead (via resolveIdent). this led me to making a patch to support ATP_PLC_HOST globally so all goat commands or other atproto tooling can operate consistently without requiring a rebuild-from-source to change defaults

[bnewbold]

I personally don't like the pattern of having env vars impact libraries in a way which is not legible or explicit to the overall process (eg, does not show up in manpage or CLI docs). This goes doubly when adding/removing/updating the behavior.

In this case I think there is a clear path for how commands can configure their own directory.

One way to make it easier to "override" inner defaults like this would be to have the default directory return a concrete type, instead of interface. Eg, DefaultBaseDirectory() which returns a BaseDirectory that can be mutated (and simply wrapped with a caching directory if desired).