Bluetooth: Mesh: Add support for manipulating Label UUIDs

Add support for sending messages that add, delete or overwrite Label
UUIDs, and add commands for these to the shell. With the help of these
commands it's possible to pass Transport Layer PTS tests (in
particular TNPT/BV-05-C) by manually adding a Label UUID through
module subscription, since the test case itself does not do this.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>