Split Repository impl by state

[Repository](https://github.com/rustic-rs/rustic_core/blob/main/crates/core/src/repository.rs) is ~2k LOC. I think it's not easy to navigate, especially given the generic implementations. I think it would be good, to split that file by the state of the implementation into submodules. E.g., `repository/open.rs`, so we know directly, that this is only containing impls for an open repository.

