Guided partitioning with multiple disks

This bug is to be further expanded with ideas, but this is a start.

It would be helpful if subiquity had an option to use a guided partitioning scheme if multiple disks are available.

For example, something simple like "()robustness ()speed", which would automatically make a choice for the user: raid1 and raid0.

The guided option should at least leave some space for the ESP partition, even if it's not populated, in order to at least facilitate recovery operations, should the disk with the ESP fail, and boot happens on the other disk without ESP. Ideally it would just work, of course, but I'm not sure what's the story with ESP on software raid1 disks.