Support installing gnome extensions

[castrojo]

GNOME Extensions are in weird spot where they come as distro packages but there's also a ton of them out there that can be installed in the userspace. It'd be nice to have a backend for installing GNOME extensions.

Right now we work around this by doing it this way: https://github.com/ublue-os/beyond/blob/main/usr/share/ublue-os/just/custom.just

This uses the awesome gnome-extensions-cli to install them on first run: https://pypi.org/project/gnome-extensions-cli/

It'd be nice if we could bundle this in yafti in a GNOME Extensions section where the user can just put in the uuid of the extension that they would like to bundle. This would allow designers to prototype out trying different extensions quickly.

[marcoceppi]

This is a great idea I'll add a gnome-extension plug-in for this

[abanna]

@marcoceppi @castrojo love it!