Add Stoat bridge

[DankRank]

Stoat (previously known as Revolt) is yet another discord replacement. I wrote this bridge back in February during the discord exodus.

List of things that work:

• Text messages
• Nicknames and avatars (both ways)
• Attachments on stoat side get turned into links

One notable limitation is that channels can only be specified by IDs, not by names. Ideally it'd be like the discord bridge, where channel names are resolved using the server id.

I don't plan on adding new features or actively maintaining it (it works fine for me and my community). Feel free to close this PR if you don't want to take this on.

[poVoq]

Can you please disclose if any LLM generated code was used to create this?

Given that we are very short on maintainers ourselves, I personally don't see this getting merged if you don't even want to do minimal maintenance of it in the future. But of course if someone else steps up that is actively using Stoat and is willing to maintain this code base then that would change things.

[DankRank]

I wrote it myself. I don't use LLMs, they're garbage.

I'm mostly posting this PR for visibility purposes, so that it can be found on the issue tracker, in case anyone wants to use it or improve on it. (That being said, this isn't a WIP branch. It should be good enough to merge if you're okay with such a minimal feature set being in master.)

I will probably keep this working for as long as it remains relevant to my community, but I'd hesitate to call myself a maintainer, as I don't really use Stoat much myself (I use XMPP), and I don't pay attention to what's going on upstream with the library or the API (though I don't expect things to break†).

†While rebasing, I noticed that revoltgo updated to golang 1.27 the day it got released, which makes me question their judgement a bit. This PR uses the last commit on 1.26. Actually, now that I look at it, they started using Claude since then. Well, fuck. 🥀