Decision to allow colon character (:) in Record Keys

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We went back and forth on this for months, but have finally decided to just allow colons (:) in record keys. We are updating the spec, and will be updating the golang and typescript SDKs soon.

The backstory on this is that the specs forbid it, but the reference PDS implementation didn't enforce the spec fully. A few popular feed generator records got created. We had concerns about potentially requiring URL encoding, or some weird syntax corner cases, but after the team read and re-read the relevant IETF RFCs several times, we decided to just chill out and relax the syntax.

We recommend folks hold off from actually creating new record keys with this character for a couple weeks, so that SDK and client app developers have time to update their software.

Relevant PRs:

• syntax: allow colon in record-key #2223
• specs: allow colon character in record-key atproto-website#296
• syntax: allow colon char in record-key indigo#591

The "interop test files" have been updated with examples.

cc: @MarshalX @myConsciousness @DavidBuchanan314 @sugyan as some SDK and lower-level devs who I can imagine might be impacted