New "clone_across_pools" volume driver capability

In at least two places (_clone_image_volume() in the create_volume flow and _get_query_filters() in the image cache) Cinder assumes that a driver cannot clone a volume into a different storage pool. Some drivers do not have such a restriction and it may be possible to complete the "create volume from image" operation very efficiently instead of falling back to the "attach and dd" option.

As discussed in the 2022-05-25 Cinder meeting, one way to fix that is to introduce a new driver capability (as reported by _update_volume_stats()) and skip the "same pool" check in these two places. The proposed new capability name is "clone_across_pools"; it will be reported as supported by the StorPool driver from the start.