Add pre/post snapshot command

It would be great to have a way to execute a command before / after creating or deleting a snapshot. This command should have as argument the name of the snapshot(s) that have been deleted.

I use btrbk to backup my SSD drive to my HDD. However, I would like to be able to boot my HDD with the last backup snapshot in case my SSD fails. To do so, I can create a writable snapshot of the HDD and replace the /etc/fstab so that it contains the HDD drive using a shell script. This script then only needs to be called by btrbk for the root subvolume with the path of the backup as an argument.

I can develop the functionality and submit a pull request if you think it would be useful to others. I am sure there are many more use cases where this can be used. I can also submit the script once it's well tested.

Start by tracing btrbk’s root-subvolume backup flow and the points where snapshots are created or deleted. Clarify the hook configuration and command arguments, including how multiple deleted snapshot names are passed; done means pre- and post-commands run reliably for both operations and the requested snapshot names are available to the script.

Written by the indexing model from the issue text.