feat: Make RatchetMapKey internal

lib/src/omemo/ratchet_map_key.dart

@@ -0,0 +1,18 @@
+import 'package:meta/meta.dart';
+
+@internal
+@immutable
+class RatchetMapKey {
+
+  const RatchetMapKey(this.jid, this.deviceId);
+  final String jid;
+  final int deviceId;
+
+  @override
+  bool operator ==(Object other) {
+    return other is RatchetMapKey && jid == other.jid && deviceId == other.deviceId;
+  }
+
+  @override
+  int get hashCode => jid.hashCode ^ deviceId.hashCode;
+}