Revert "make identity.Identity more easily JSON serializable" (#1037)
This caused problems in prod. Unbeknownst to me, we had been serializing
identity metadata as JSON using this struct and persisting it. Then
passing back to TypeScript (not golang!), which depended on the
capitalization (case-sensitive JSON parsing).
Reverting for now. Note that the go stdlib JSON code is actually
case-insensitive for keys, so the text fixtures in this repo which had
been updated to depend on this are not impacted (!).
Direct link to original PR:
https://github.com/bluesky-social/indigo/pull/1017