Bluesky: DM bot user to prompt remote users to enable (opt into) the bridge

[snarfed]

Blocking this on Bluesky OAuth since I want to identify the requesting Bluesky user and include their handle in the prompt DM, but I don't want to make them enter an app password. Bluesky team says OAuth is "a few weeks away," bluesky-social/atproto#2428 (comment), so we can wait.

[snarfed]

"A few weeks away" was a bit ambitious on their part. 😆 Also, interestingly, Bluesky shipped DMs before OAuth, so they have parity with the fediverse on that count. Given all that, I'm going to pivot to using DMs for the opt in prompt on Bluesky as well as in the fediverse. Renaming this issue to track that.

[snarfed]

In short, the idea here is that Bluesky users should be able to request bridging a fediverse user by DMing their handle to @ap.brid.gy@ap.brid.gy. BF will then DM that user in the fediverse, and they can respond yes or no.