Add AppImage Update Metadata and .zsync Support

It would be useful if the AppImage included the required update metadata so AppImage managers such as AppManager can detect new releases and support automatic updates.

For example, the AppImage desktop entry could include:

X-AppImage-Homepage=https://github.com/OWNER/REPOSITORY
X-AppImage-Name=APP_NAME
X-AppImage-Version=VERSION
X-AppImage-Arch=x86_64
X-AppImage-UpdateURL=https://github.com/OWNER/REPOSITORY

The X-AppImage-Version value should be generated automatically from the application version or release version at build time rather than being hardcoded.

Additionally, for compatibility with AppImageUpdate, it would be useful to provide a .zsync file for each AppImage release and include the appropriate update information inside the AppImage, for example:

gh-releases-zsync|OWNER|REPOSITORY|latest|APP_NAME_*_amd64.AppImage.zsync

The corresponding .AppImage.zsync file should also be published alongside the AppImage as a release artifact.

This would allow AppImage managers to properly identify the application, detect new versions, and update the AppImage efficiently.

Thanks!