Design future improvements to backups

In the past years, we didn't prioritize working on backups from the core team. It was still a very hot topic, so as a result, other people started pushing for incremental changes to improve the situation: @david-a-wheeler in #18504 (closed), @segfault in #7049 (closed), and @BenWestgate in a bunch of places.

I'm very excited to see that other people are contributing very important improvements outside of the limitations of the core team, but I feel like we're reaching a point where all this would need a bit more coordination, deeper thoughts, less context-switching, and more upfront design. For example, right now tails-backup and tails-cloner feel a bit like duplicates, they work independently from tails-persistent-storage, etc.

So, before we start working on another bit of backup tooling, I'd propose @foundations-team to sit down with me design better the broader picture of what backup system we want in the end. Once we have a plan, we could still make it possible for other people to implement this plan bit by bit, but with more coordination.

We're aiming at 2024Q2 for that.

Version 3 from 2024-09-04

Pending cost and strategic decision on whether to integrate everything in the Persistent Storage settings (@segfault's mockups) or adapt Tails Cloner (@sajolida's and @anonym's mockup).