Add dm-thin support

thin-pool and thin targets are supported via DmTargetThinPool and
DmTargetThin. DM_TARGET_MSG is also added via a new method
SendMessage() because it's used to create a thin volumn.

dmctl is extended to support thin-pool and thin targets.

TODO: thin-pool target constructor can accept feature arguments.

Bug: 327081431
Test: atest libdm_test (CF with dm-thin enabled kernel)
Change-Id: I4c51c668bfe1489b959f6d03c205a5e2e63d9a1d