Provide data safe key import facilities

The nethsm crate currently relies on key import facilities that are not data safe: https://gitlab.archlinux.org/archlinux/signstar/-/blob/85c699dafb91afa0721e404f22150647e0b36082/nethsm/src/nethsm_sdk.rs#L111-184

We need to provide a cleaner interface for key data import and allow the distinction between different key types.