Allow dm-cache over luks devices

[axet]

dm-cache can speed up devices read access for slow devices like usb flash or hdd. Perfomance could be improved using dm-cache layer. It has to be manually called using:

echo 0 131072 cache /dev/sdc /dev/sdb 0 8 65536 256 1 | dmsetup create foo

Would be nice to have udisks2 option for auto caching mounted and unlocked LUKS devices via dm-cache paritition / file.

For example having default dm cache parititon pool which can be configured using udisks2 configs which can be automatically pulled or created (using files in target directory on SSD).