postgres: add --label flag

[TLINDEN]

The schema already supports labels, I am able to add one via cloudctl postgres edit $uid.

Now if cloudctl postgres ls had a --label parameter (just like cloudctl cluster ls already has) would be very useful. That way one could mark postgres instances and then filter them based on those labels.

My current usecase would be to mark an instance as "the original one" and then update it (which creates a clone). When everything is ok, it would be easyer to distinguish the original one from the clone. Another usecase would be to mark instances as "to be deleted soon" or "do not update" or whatever.