[FEATURE REQUEST] Add custom sources à la Obtainium for Android #3187

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use this template.

Describe the new feature

Bon Dia Marti, UnIget Community!

I was wondering if there is a way of adding custom sources not included in any package manager directly through Uniget. The feature has been suggested before in #2233, and follows the main feature of Obtainium for Android, in which the source and release link is specified (mostly GitHub/GitLab) and the software checks against the local installation for updates.

As an alternative, I am adding custom manifests to Scoop that get updated through Uniget, but I would like a more direct mechanism so that I can leave scoop installs only for cli tools.

Any solution is kindly welcomed.

Describe how this new feature could help users

There is plenty of software and source code that does not make it to curated package managers. Curious users and power users that rely on unpopular pieces of code and software would benefit from the sometimes infrequent updates of these packages if they could be added through a custom source specification and get checked for updates without having to visit, ever so rarely, the sources to check for updates. It could be a great power feature, specially if it could also check not only for binaries but also source code releases.