atproto/data: package for working with generic schema-less atproto data in JSON or CBOR

[bnewbold]

So far most golang atproto code has either worked with data in a known schema, or entirely ignored the contents. We will at times need to convert to and from JSON and CBOR when a schema isn't available. Or even when the schema is available, a record might have unexpected additional fields, and conversion might need to be done in a way that preserves all fields.

Reused some existing test fixtures to ensure interop and consistent behavior against typescript implementation.

The Bytes, Blob, and CIDLink structs are copied from lex/util, and could replace those implementations.

Expect to implement atproto/lexicon on top of this package to do run-time schema validation.

"Extract all blobs from unknown data" is a thing that the distributor or appview v2 may need to do in the near future, to copy in to CDN and run image auto-moderation.

I bet there is a much more efficient way to implement the "parsing" functions. The goal with this initial version of the package is to get a reasonable API and test coverage of weird corner-cases. Should be able to optimize the implementation later.