Support Forgejo as a standalone option (instead of via Gitea)

[filipdutescu]

As a user of Forgejo, I want to be able to use a repository from a forge to be used by Flux. Forgejo is a (now) hard-fork of Gitea which aims to preserve compatibility with it, as much as possible.

Hopefully, this means it should not be too hard to add support for Forgejo and that, at least for the foreseeable future, it should not be too much of a burden, as Gitea is already supported.