RUST: Add functions in the new db to add/delete karas

inc/liblektor-rs/database.h

src/rust/liblektor-rs/src/database/error.rs

src/rust/liblektor-rs/src/database/mod.rs

src/rust/liblektor-rs/src/database/models.rs

src/rust/liblektor-rs/src/database/unsafe_interface.rs