[Pure Storage] Enable tri-sync replication

Enable trisync replication capability for Pure Storage volumes.

Trisync is the ability to replicate a volume synchronously to a target array and also have an asynchronous copy also be created on another target array.

Note that for testing purposes the target array can be the same for both the sync and async legs.

This feature can be enabled by setting the parameter in the backend stanza

pure_trisync_enabled = True

The default will be False

To enable tri-sync successfully there must be 2 replication_devices configured in the stanza, one that is type=sync and one that is type=async.
Multiple versions of either sync or async will not allow trisync to be enabled as we would not know which replication targets or AC pods to use with tri-ync.
Enable retyping between sync and trisync