Make share shrinking support a capability

Manila supports end users discovering capabilities with the help of tenant-visible extra specs. This approach makes it very easy for cloud administrators (that understand their backend storage systems) to provide a storage service catalog on their clouds.

Shared file systems are inherently elastic, however, there are many shared file system storage providers in the wild that do not support "shrinking" file shares. So, it is not a "required" feature to implement on storage drivers within manila. See the feature support martix for the Rocky cycle here: https://docs.openstack.org/manila/rocky/admin/share_back_ends_feature_support_mapping.html

Currently there is no way for end users to know before hand if the storage backend their shares end up on will support shrinking. If we included "shrinking_support" as a capability, we would be able to reject share shrinking requests via the API directly. The UI could discover if a given share was provisioned on a backend that supported shrinking shares, and disallow shrinking to lend a better user experience.