encodeIss method
Encodes the public key into a multicodec issuer
Implementation
@override
String encodeIss(Uint8List publicKey) {
  Uint8List header = base58.decode(multicodecEd25519Header);
  final String multicodec =
      '$multicodecEd25519Base${base58.encode(Uint8List.fromList(header + publicKey))}';
  return <String>[DID_PREFIX, DID_METHOD, multicodec].join(DID_DELIMITER);
}