feat: Expose rotating our own bundle's SPK

lib/src/omemo/sessionmanager.dart

@@ -369,6 +369,19 @@ class OmemoSessionManager {
 
     return fingerprints;
   }
+
+  /// Replaces the Signed Prekey and its signature in our own device bundle. Triggers
+  /// a DeviceModifiedEvent when done.
+  /// See https://xmpp.org/extensions/xep-0384.html#protocol-key_exchange under the point
+  /// "signed PreKey rotation period" for recommendations.
+  Future<void> rotateSignedPrekey() async {
+    await _deviceLock.synchronized(() async {
+      _device = await _device.replaceSignedPrekey();
+
+      // Commit the new device
+      _eventStreamController.add(DeviceModifiedEvent(_device));
+    });
+  }
   
   @visibleForTesting
   OmemoDoubleRatchet getRatchet(String jid, int deviceId) => _ratchetMap[RatchetMapKey(jid, deviceId)]!;

test/omemo_test.dart

@@ -221,4 +221,28 @@ void main() {
     // This call must not cause an exception
     bobSession.getRatchet(aliceJid, (await aliceSession.getDevice()).id);
   });
+
+  test('Test rotating the Signed Prekey', () async {
+    // Generate the session
+    const aliceJid = 'alice@some.server';
+    final aliceSession = await OmemoSessionManager.generateNewIdentity(aliceJid, opkAmount: 1);
+
+    // Setup an event listener
+    final oldDevice = await aliceSession.getDevice();
+    Device? newDevice;
+    aliceSession.eventStream.listen((event) {
+      if (event is DeviceModifiedEvent) {
+        newDevice = event.device;
+      }
+    });
+
+    // Rotate the Signed Prekey
+    await aliceSession.rotateSignedPrekey();
+
+    // Just for safety...
+    await Future<void>.delayed(const Duration(seconds: 2));
+
+    expect(await oldDevice.equals(newDevice!), false);
+    expect(await newDevice!.equals(await aliceSession.getDevice()), true);
+  });
 }