Find a way to expand /system virtual partition from recovery, for devices which use super partition

We're going to include Qt 6 alongside Qt 5 in our image for the benefit of Morph Browser having newer QtWebEngine/Chromium. But this means increased space usage while space is already limited on many devices.

It's been found that, for many devices, there's actually plenty of room in super partition left unallocated. This is because the size of /system virtual partition was arbitrary selected when the need to resize /system virtual partition first arose.

This resizing is normally done by UBports Installer, with size being specified in installer-config. We don't want users to go through UBports Installer on a computer again to upgrade, so instead we want to do this from recovery.

This is definitely possible; fastbootd, the component which do the actual virtual partition resize, runs in essentially the recovery environment. We probably want to tap into that somehow.