"Save a copy to" action to easily copying a vault in a new storage directory (backup)

### Please agree to the following

- [x] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [x] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Save a copy of a preexistant (local) vault in a usb dongle (for example).

### Motivation

Doing this manually can expose the user to some dangers, like deleting mistakenly a vault if one taps "move" (or "delete") instead of "copy" in a file manager.

For this reason many password managers have this feature built-in.

### Considered Alternatives

_No response_

### Anything else?

_No response_