Implement optional API for shrinking shares. This would add an API call to specifically shrink the size of an existing share that is currently in available state. The call would be similar to extend API: "manila shrink <share-id> <new-size>" https://blueprints.launchpad.net/manila/+spec/share-shrink-api https://blueprints.launchpad.net/manila/+spec/shrink-share-in-generic-driver