Add the ability to wait for the backup operation to finish and print the result

The flag will wait until a backup is finished. Reasons can be: error, done, canceled.

example output:

pbm backup -w

Starting backup '2022-06-08T07:40:04Z'....
Waiting for '2022-06-08T07:40:04Z' backup....... done

Without it, the command just waits till backup is started. It still can be in progress when the command returns