[FEATURE REQUEST] - add update metadata to the appimage

[DetachHead]

What problem does the feature request solve? instead of having its own auto update system (which causes issues like #1037), r2modman should include an update url in its metadata so that tools like gear lever can be used to manage updates instead

Example mock-up (optional) If you are able to provide a mock-up, please do so here

Additional information (optional) i'm no expert but i believe this is the documentation that explains how to do this