doc: Collection object and metadata spec

[ShrimpCryptid]

Specification updates for the new collection objects and metadata! This doesn't include any implementation support yet; I wanted to get a gut check before moving forward with exact implementation details.

(This and the subsequent PRs will hopefully all get merged at once so there's never a mismatch between the documentation and the actual writer behavior.)

Changes

• Updates DATA_FORMAT.md with the new planned metadata for collections and datasets.
• Adds new base types for shared metadata.
• Adds new dictionary types for the collection file.

Estimated review size: medium, 20-30 minutes

[ShrimpCryptid]

Thank you both for reviewing! Going to leave this open until the next two PRs are approved so we don't end up with a desync between the documentation and the behavior.