[Feature] Add `swaync-client --list-notifications`

[unikitty37]

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Please also make sure that there isn't any prior issue describing this feature

Is your feature request related to a problem? Please describe.
When I receive a notification that I need to write a rule for (example: satty sends its notifications as normal urgency rather than low) I cannot easily get the app-name or other notification details to match against.

Describe the solution you'd like
With the notification still on the screen or in the control centre, I'd like to type swaync-client --list-notifications and get all the information I need about the notification that is already on my screen.

Describe alternatives you've considered
The only way to work around this at the moment seems to be:

1. Quit swaync
2. Run G_MESSAGES_DEBUG=all swaync
3. Try to get the program that generated the notification to do it again (and this is not always easy!)
4. Get the information I need and save it somewhere
5. Get swaync back running in the background again — since niri spawns this at login, that means logging out and back in again unless I want to leave nohup.out files lying around the place or dedicate a terminal window to swaync.
6. Finally write the rule

An alternative would be for swaync, if run with G_MESSAGES_DEBUG=all and there is already a swaync instance running, to run and show the debug output without trying to do anything else — the running instance would handle showing the notifications, and the debugging instance would just show messages. Even then, though, I would still need to be able to reliably retrigger the notification, so this isn't an ideal solution compared to having swaync-client just output the data that swaync presumably still has in memory somewhere.

Additional context