schema-agnostic API helpers

[bnewbold]

These are some variants of "typed" atproto sync/repo helpers from api/atproto which use json.RawMessage instead of the lexutil LexiconTypeDecoder wrapper.

These are mostly used from tools like goat which work with arbitrary record data, when we don't have lexgen types (or they aren't imported, or whatever). They can also be used for things like preferences, where the type in an open union is rapidly changing, and we don't want to "clobber" (drop) data from new lexicons the client code isn't familiar with.

You can see these in action: #870

@whyrusleeping you might want to do similar things in gosky.

Big picture we'll maybe want to move lexgen in this direction at some point? Relatedly, having LexiconTypeDecoder and open unions have an "unknown schema" option with raw data (possibly map[string]any instead of json.RawMessage; the later only makes sense for API endpoints, not record data).

[brianolson]

double check the comments on where things were copied from, a couple were bsky not atproto, otherwise makes sense

[brianolson]

api/bsky (in a few of these files)