[FR] add support for gitlab and other platforms

[Samueru-sama]

The original appimageupdate only has support for gh-releases-zsync, which makes it search for the latest zsync release on github with this format:

gh-releases-zsync|<username>|<repo>|<tag>|<filename> (with filename supporting simple globbing).

For other platforms, people instead have to use zsync|<url> which is very problematic, because it has to be a fixed url that projects have to setup.

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Would be great to also have gl-releases-zsync for gitlab, cb-releases-zsync for codeberg and so on. Which would do what gh-releases-zsync does but on those platforms.