Add Flatpak support

It would be nice to have the option to install Hydra as a proper Flatpak and for it to support Flatpak installs of Wine and Lutris.

Since you already support snap, it would be natural to use Flatpak, since it’s far more widespread and used than snap.

(For atomic distros like Fedora Silverblue, this is vital to be able to use Hydra in the first place)