feat: Allow calculating fingerprints

lib/src/omemo/fingerprint.dart

@@ -0,0 +1,19 @@
+import 'package:meta/meta.dart';
+
+@immutable
+class DeviceFingerprint {
+
+  const DeviceFingerprint(this.deviceId, this.fingerprint);
+  final String fingerprint;
+  final int deviceId;
+
+  @override
+  bool operator ==(Object other) {
+    return other is DeviceFingerprint &&
+      fingerprint == other.fingerprint &&
+      deviceId == other.deviceId;
+  }
+
+  @override
+  int get hashCode => fingerprint.hashCode ^ deviceId.hashCode;
+}