feat(backend)!: Add backend to config and use `rustic_backend`

[simonsan]

BREAKING CHANGE:

• Config structure changes

TODO:

• [ ] how to deal with copy in a good way
• [ ] decide on toml table names and structure, I would prefer it as storage and parse the provider, as in here:

- # Backend options: These options define which backend to use.
- [backend]
- repository = "/tmp/repo"
+ # Storage options: These options define which backend to use.
+ [storage]
+ provider = local
+ path = "/tmp/repo"