atproto/lexicon: package for working with lexicon schemas, and runtime data validation

[bnewbold]

This is currently a branch on top of #407

• parse lexicon schema JSON
• load entire directories of schema JSON files from disk as a catalog
• check lexicon schema semantics (eg, can't have min greater than max)
• validate runtime data (map[string]any) against lexicons
• whole bunch of corner-case tests
• CLI tool for some live-network testing
• CLI tool to validate prod firehose
• CLI tool to validate CAR files
• configurable flexible to legacy blobs and lenient datetime parsing (?)
• clarify specs around unions: only object and token types?
• clarify specs around unknown: only object type?
• add support for tid and record-key lex formats (not in specs yet)
• validate rkey type against lexicon

probably in a later iteration:

• validate other "primary" lexicon types: subscription, HTTP body, HTTP URL params, etc