x/build/cmd/relui: add support for making release announcement posts on Bluesky

[dmitshur]

During Go releases, relui takes on posting on 2 social sites using their APIs (#47403 and #65046). By now we have an account on one more social site, Bluesky, where we've also started to post links to Go release announcements (for example, https://bsky.app/profile/golang.org/post/3lfvcgeox7c2c).

This is the tracking issue for automating Go release announcement posts on Bluesky.

CC @golang/release, @mknyszek, @fflewddur.

[fflewddur]

Thanks @dmitshur! I noticed this was working when I went to manually share the latest Go release on Bluesky, but found it had already been posted. It's great to see this automated!

[dmitshur]

@fflewddur To clarify, that time when you saw it already posted, it must've been posted manually by someone else. The work of automating this hasn't been done yet, but this issue tracks progress on doing exactly that.