Support authenticated Flatpak apps

[AsciiWolf]

Flatpak supported authenticated downloads for a while now, however if I am not mistaken, the front-end support was never added to Bazaar. Please, consider adding it. It could be used for paid commercial software downloads or enterprise use cases where secure, authenticated channel is needed. Simple auth dialog and some basic code using the available (lib)flatpak API should be sufficient. Please see links in the "More details" below.

More details:
flatpak/flatpak#3167
https://github.com/flatpak/flatpak-sample-authenticator
https://github.com/flatpak/flatpak/blob/b8d8e5bf043476364f496170b46695a7a42c7e57/common/flatpak-transaction.h#L135

Other tickets:
GNOME - https://gitlab.gnome.org/GNOME/gnome-software/-/work_items/3016
KDE - https://bugs.kde.org/show_bug.cgi?id=524193

[AsciiWolf]

Support in GNOME Software was added in 2020:

https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/467