Add the org.gnome.Games.LibretroPlugin.PicoDrive Flatpak extension whose
content will appear in /app/lib/libretro/extra.d/PicoDrive.

This allows to play Master System, Mega Drive/Genesis (and add-ons) and
Pico games.

It is added as an extension because PicoDrive is currently licensed
under the non-free MAME license.