dissect: look for new GPT partition flag marking partitions for growing

dissect: look for new GPT partition flag marking partitions for growing

systemd-repart can grow partitions dynamically at boot, but it won't
grow the file systems inside them. In /etc/fstab you can request that
via x-systemd.growfs. So far we didn't have a nice scheme for images
with GPT auto-discovery however, and that meant in particular in tools
such as systemd-nspawn the file systems couldn't be grown automatically.

Let's address this: let's define a new GPT partition flag that can be
set for our partition types. If set it indicates that the file system
should be grown to the partition size on mount.

This commit adds the flag and adds code to discover it when dissecting
images. There's no code yet to actually do something about it.