feat: Help with serializing and deserializing the BTVT manager

lib/src/omemo/ratchet_map_key.dart

@@ -4,9 +4,24 @@ import 'package:meta/meta.dart';
 class RatchetMapKey {
 
   const RatchetMapKey(this.jid, this.deviceId);
+
+  factory RatchetMapKey.fromJsonKey(String key) {
+    final parts = key.split(':');
+    final deviceId = int.parse(parts.first);
+
+    return RatchetMapKey(
+      parts.sublist(1).join(':'),
+      deviceId,
+    );
+  }
+
   final String jid;
   final int deviceId;
 
+  String toJsonKey() {
+    return '$deviceId:$jid';
+  }
+  
   @override
   bool operator ==(Object other) {
     return other is RatchetMapKey && jid == other.jid && deviceId == other.deviceId;