Feature Request: Named targets

Extending https://github.com/digint/btrbk/issues/356, targets should have an option to have an alias. This will bring the ability to:
1. Disable all targets temporarily,
2. Enable some of the targets (present/reachable)
3. Enable all of the targets

So we could keep only one configuration file for multiple backup disks/locations and override options to skip some of them due to the temporary time constraints or target presence.

Start by reviewing issue #356, which this request extends, and clarify how named target aliases should be configured and selected. Confirm that the finished behavior supports temporarily disabling all targets, enabling only present or reachable targets, and enabling all targets while allowing overrides.

Written by the indexing model from the issue text.