feat: Allow calculating fingerprints

test/double_ratchet_test.dart

@@ -79,11 +79,13 @@ void main() {
     // Build a session
     final alicesRatchet = await OmemoDoubleRatchet.initiateNewSession(
       spkBob.pk,
+      ikBob.pk,
       resultAlice.sk,
       resultAlice.ad,
     );
     final bobsRatchet = await OmemoDoubleRatchet.acceptNewSession(
       spkBob,
+      ikAlice.pk,
       resultBob.sk,
       resultBob.ad,
     );

test/omemo_test.dart

@@ -132,5 +132,15 @@ void main() {
       bobResponseMessage.encryptedKeys,
     );
     expect(bobResponseText, aliceReceivedMessage);
+
+    // Alice checks the fingerprints
+    final fingerprints = await aliceSession.getHexFingerprintsForJid(bobJid);
+    // Check that they the fingerprints are correct
+    expect(fingerprints.length, 2);
+    expect(fingerprints[0] != fingerprints[1], true);
+    // Check that those two calls do not throw an exception
+    aliceSession
+      ..getRatchet(bobJid, fingerprints[0].deviceId)
+      ..getRatchet(bobJid, fingerprints[1].deviceId);
   });
 }