feat: Add last features for use in moxxmpp

lib/src/double_ratchet/double_ratchet.dart

@@ -1,5 +1,6 @@
 import 'dart:convert';
 import 'package:cryptography/cryptography.dart';
+import 'package:hex/hex.dart';
 import 'package:meta/meta.dart';
 import 'package:omemo_dart/src/crypto.dart';
 import 'package:omemo_dart/src/double_ratchet/crypto.dart';
@@ -251,6 +252,12 @@ class OmemoDoubleRatchet {
       'kex': kex,
     };
   }
+
+  /// Returns the OMEMO compatible fingerprint of the ratchet session.
+  Future<String> getOmemoFingerprint() async {
+    final curveKey = await ik.toCurve25519();
+    return HEX.encode(await curveKey.getBytes());
+  }
   
   Future<List<int>?> _trySkippedMessageKeys(OmemoMessage header, List<int> ciphertext) async {
     final key = SkippedKey(