Add support for long-running restic commands

[oliver-la]

Just remembered I never created a feature request for this. At last, here it is. :)

Is your feature request related to a problem? Please describe. Backrest currently runs custom commands in the foreground/browser and cancels the execution when the browser is closed or the connection interrupts. It's therefore unsuitable to run commands that may take a long time. For example, I'd like to run prune with extra flags which is going to take quite some time.

Describe the solution you'd like The commands should be sent to the background. A new entry both in the Tree View and Operation List of the repository should appear, with a visual indication of the command's status. (running/ended/failed) As a user I should also be able to view the output of the command.

Or, as Gareth put it:

My current idea is to eventually introduce a new operation type + task type for custom commands that will be tracked in the operation log (and support all the normal task features like cancellation, output and log tracking, etc). I'll likely implement some small condensed display for recently run custom command output in the Run Command view.

Discussed in https://github.com/garethgeorge/backrest/discussions/482

^{Originally posted by oliver-la September 19, 2024} The web ui currently allows to run a custom restic command, with the caveat that it's only intended for commands that complete in a short time.

I'd like to run a prune with a couple extra flags. Since it will take a long time, I probably(?) can't do it via the Web UI.