Plugin hook for user registration

[Nutomic]

It would be useful to add a plugin hook for new user registration, particularly when registration approval is enabled. The plugin would be called with the answer and other data as parameter, and immediately returns approve or deny if the answer contains specific words. Some instances like lemmy.world already do this, but it goes through the api and needs an extra step to send approve/deny via email. With a plugin it can immediately login the user on approve, or directly show the deny message on the registration page.

[Nutomic]

Similarly a plugin hook for approval for private community followers. These can be automatically approved or rejected based on profile content, account age or comment count.

And a plugin hook for federation with new instances (https://github.com/LemmyNet/lemmy/issues/1487, https://github.com/LemmyNet/lemmy/issues/3953, https://github.com/LemmyNet/lemmy/issues/5088)