Linux: Embed AppImage update information

Request to provide `.zsync` file for AppImage delta updates.
Updating will take less amount of data and time due to downloading only `diff`.

info:
- https://github.com/AppImageCommunity/AppImageUpdate#update-information-overview
- https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information
- https://github.com/ivan-hc/AppImage-tips#how-to-add-delta-updates

This is a common standard for updating AppImages.
AppImage having update information would also recognized by other software e.g. [AppImageLauncher](https://github.com/TheAssassin/AppImageLauncher), [AppImageUpdate](https://github.com/AppImageCommunity/AppImageUpdate), [AppManager](https://github.com/kem-a/AppManager), [AM](https://github.com/ivan-hc/AM) etc...

Lots of projects already support updating AppImage e.g: [LibreWolf](https://librewolf.dev/librewolf/bsys6/releases), [KeePassXC](https://github.com/keepassxreboot/keepassxc/releases), [Sigil](https://github.com/Sigil-Ebook/Sigil/releases) etc.

Start by locating the Linux AppImage packaging and release entry points; no files or tests are named in the issue. Read the linked AppImage update-information specifications and compare existing release artifacts from the referenced projects. Done means the published AppImage has usable .zsync update information that AppImageUpdate and related software recognize.

Scritto dal modello di indicizzazione a partire dal testo della issue.