ffi(rust): add view-only capabilities

This crate should cover a slice of the functionality, enough to allow a view-only (UFVK for now) wallet to function.

More precisely, the FFI should:

- Allow the creation of a lightwallet and lightclient from a given UFVK string,
- Expose sync functionality (start, stop, poll, report)
- Query balance

Another concern is file storage. This will need to be done in whichever PR lands first.