Allow share backup driver to pass external backup / restore progress metrics for periodic share-backup tasks

Currently, for periodically fetching the progress of backup and restore operations that have an in progress state for user ui feedback / for erroring stale operations, the data manager utilises a `self.data_copy_get_progress` function that internally recovers this information.

No such concept exists in the base class for Manila backup drivers `backup_driver.py`, meaning a driver that has an external mechanism to Manila for determining restore / backup progress cannot be a source for providing operation progress.

This Blueprint proposes to expand the BACKUP_API and `backup_driver.py` so that this internal operation is exposed to the base class of the backup driver by including a `get_progress` definition along with changes to the data manager functions:
`create_backup_continue`
`restore_backup_continue`

that makes them utilise the driver for this operation when the `self.use_data_manager` boolean is unset (identical to what has been covered in https://blueprints.launchpad.net/manila/+spec/share-backup-use-boolean)