feat: Reimplement the OMEMOMessage protobuf schema

lib/src/protobuf/omemo_message.dart

@@ -0,0 +1,72 @@
+import 'package:omemo_dart/src/helpers.dart';
+import 'package:omemo_dart/src/protobuf/protobuf.dart';
+
+class OmemoMessage {
+
+  const OmemoMessage(this.n, this.pn, this.dhPub, this.ciphertext);
+
+  factory OmemoMessage.fromBuffer(List<int> data) {
+    var i = 0;
+    
+    // required uint32 n          = 1;
+    if (data[0] != 8) {
+      throw Exception();
+    }
+    var decode = decodeVarint(data, 1);
+    final n = decode.n;
+    i += decode.length + 1;
+
+    // required uint32 pn         = 2;
+    if (data[i] != 16) {
+      throw Exception();
+    }
+    decode = decodeVarint(data, i + 1);
+    final pn = decode.n;
+    i += decode.length + 1;
+
+    // required bytes  dh_pub     = 3;
+    if (data[i] != ((3 << 3) | 2)) {
+      throw Exception();
+    }
+    final dhPub = data.sublist(i + 2, i + 2 + data[i + 1]);
+    i += 2 + data[i + 1];
+
+    // optional bytes  ciphertext = 4;
+    List<int>? ciphertext;
+    if (i < data.length) {
+      if (data[i] != ((4 << 3) | 2)) {
+        throw Exception();
+      }
+
+      ciphertext = data.sublist(i + 2, i + 2 + data[i + 1]);
+    }
+
+    return OmemoMessage(n, pn, dhPub, ciphertext);
+  }
+
+  final int n;
+  final int pn;
+  final List<int> dhPub;
+  final List<int>? ciphertext;
+
+  List<int> writeToBuffer() {
+    final data = concat([
+      [8],
+      encodeVarint(n),
+      [16],
+      encodeVarint(pn),
+      [((3 << 3) | 2), dhPub.length],
+      dhPub,
+    ]);
+
+    if (ciphertext != null) {
+      return concat([
+        data,
+        [((4 << 3) | 2), ciphertext!.length],
+        ciphertext!,
+      ]);
+    }
+
+    return data;
+  }
+}