flat-manager: Support profiles

[GeorgesStavracas]

(Draft: this is just the skeleton, still needs to fill in the validation function)

Add support for profiles, and add the 'flathub' profile.

Related: https://github.com/bilelmoussaoui/flatpak-github-actions/issues/64

[bilelmoussaoui]

The issue we have right now is we need such option for flatpak-builder as well so we can automatically figure out where to mirror the screenshots; otherwise you will need to specify it manually which I think is not great :/