Add collection parameter to com.atproto.sync.getRepo

[DavidBuchanan314]

It should be possible for a PDS (or relay) to efficiently serve a slice of the repo CAR that contains

a) all the records in a collection
b) all the MST blocks needed to verify them

This would be especially useful for anyone trying to backfill the whole network, but who only cares about a particular collection (say, follows)

It should also be possible to combine the since and collection parameters (although I haven't thought about how I'd implement it yet, maybe there's some hidden complexity there I haven't considered)

[DavidBuchanan314]

Bonus feature - allow a single wildcard at the end of the collection parameter, e.g. app.bsky.* (this is efficient to answer since the matching collections are guaranteed to be adjacent in the MST)