Updates to blob behaviors: `$type`, cid encoding, etc

[pfrazee]

• Update all blob usages (values of type blob, image, video, audio) to include a $type field with the relevant type included
  • Enforce this during write validation but not during reads
• When translating blobs to CBOR, encode the cid field to the link tag when possible

[dholms]

fixed https://github.com/bluesky-social/atproto/pull/658