gnutls_privkey_import_ext2: ensure that it can operate with RSA-PSS and Ed25519
Our APIs for external keys are quite limited and were designed for ECDSA and RSA which can operate with arbitrary hash algorithms. As we are moving towards keys which support only particular signature-algorithms, and also keys which can only operate on raw data (not hashed), we should update the external key APIs to be able to cope with that.