Add Gitea support

[noerw]

Gitea is a self hosted git hosting solution, quite similar in functionality (and URL structure) to GitHub.
Would be nice if repos hosted on a Gitea instance would be supported by this plugin.

One way to detect an Gitea instance would be to query <baseurl>/api/v1/nodeinfo, where baseurl is the git remote value minus the last 2 path levels (eg example.com/gitea/foo/bar → example.com/gitea (example)

Alternatively, there are a couple of bigger Gitea instances which this plugin could hardcode:

• https://codeberg.org (public non-profit FOSS hosting)
• https://gitea.com (gitea development)

If you can point me to resources how to add support for this, I could take on the implementation myself.