feat: implement rustic support

[garethgeorge]

This is largely an experimental exploration into rustic support https://github.com/rustic-rs/rustic

[ilkersigirci]

This is huge, when do we see this in the release? Is there any roadmap, thanks

[garethgeorge]

Thanks for the status ping on this, it's still something I'm interested in but got lost in the backlog of backrest work :).

I think my current thinking is to re-evaluate whether I want to create a gRPC daemon that wraps rustic (which is a pretty big maintenance burden unless this is something the rustic project can provide). But I think it should be reasonable for me to wrap rustic's cli (which IIUC already has some json output?) and see how far I can get with that.