Expose the PDS/instance's network affiliation via an endpoint

[intrnl]

Is your feature request related to a problem? Please describe.

Client apps would like to know which network they are in, whether it's production, sandbox, or other potential networks, this is so they can safely connect to adjacent services that taps into Bluesky, like ~~jcsalterego's goodfeeds or~~ the production network's search service (search.bsky.social)

Describe the solution you'd like

An endpoint that returns their network affiliation, like their PLC provider, BGS, or App View service

Describe alternatives you've considered

The current solution for clients right now are the following:

• Assume we're on production network if we're on bsky.social, dev network if localhost, and sandbox for the rest
• Check if a certain thing like a post or profile exists, if so, we're on the production network, if not, well, who knows? unless one wants to make a unique post for each network.

Additional context

None.

[snarfed]

This could probably just be a new field in the com.atproto.server.describeServer response.