Generate nicer return structs

You can define structs to map types to in zbus: https://dbus2.github.io/zbus/client.html It'd be nice if e.g. `list_units` returned a Vec like: ```rust struct Unit { name: String, description: String, load_state: String, active_state: String, sub_state: String, followed_unit: String, path: OwnedObjectPath, job_id: u32, job_type: String, job_object_path: OwnedObjectPath, } ``` Instead of the Vec of tuples we have right now