Add progress reporting to cbbackupmgr via REST

At the moment the only way to monitor how far along a backup/restore is, is to look at the progress bar, this is not ideal for automation or if you are managing multiple clusters simultaneously. It also means that when we give cbbackupmgr a UI there will be no way to report progress. This is why adding a way cbbackupmgr to report its progress via REST will come in handy.