feat: Implement all protobuf messages

lib/src/protobuf/omemo_key_exchange.dart

@@ -0,0 +1,67 @@
+import 'package:omemo_dart/src/helpers.dart';
+import 'package:omemo_dart/src/protobuf/omemo_authenticated_message.dart';
+import 'package:omemo_dart/src/protobuf/protobuf.dart';
+
+class OmemoKeyExchange {
+
+  const OmemoKeyExchange(this.pkId, this.spkId, this.ik, this.ek, this.message);
+
+  factory OmemoKeyExchange.fromBuffer(List<int> data) {
+    var i = 0;
+
+    if (data[i] != fieldId(1, fieldTypeUint32)) {
+      throw Exception();
+    }
+    var decoded = decodeVarint(data, 1);
+    final pkId = decoded.n;
+    i += decoded.length + 1;
+
+    if (data[i] != fieldId(2, fieldTypeUint32)) {
+      throw Exception();
+    }
+    decoded = decodeVarint(data, i + 1);
+    final spkId = decoded.n;
+    i += decoded.length + 1;
+
+    if (data[i] != fieldId(3, fieldTypeByteArray)) {
+      throw Exception();
+    }
+    final ik = data.sublist(i + 2, i + 2 + data[i + 1]);
+    i += 2 + data[i + 1];
+
+    if (data[i] != fieldId(4, fieldTypeByteArray)) {
+      throw Exception();
+    }
+    final ek = data.sublist(i + 2, i + 2 + data[i + 1]);
+    i += 2 + data[i + 1];
+
+    if (data[i] != fieldId(5, fieldTypeByteArray)) {
+      throw Exception();
+    }
+    final message = OmemoAuthenticatedMessage.fromBuffer(data.sublist(i + 2));
+
+    return OmemoKeyExchange(pkId, spkId, ik, ek, message);
+  }
+  
+  final int pkId;
+  final int spkId;
+  final List<int> ik;
+  final List<int> ek;
+  final OmemoAuthenticatedMessage message;
+
+  List<int> writeToBuffer() {
+    final msg = message.writeToBuffer();
+    return concat([
+      [fieldId(1, fieldTypeUint32)],
+      encodeVarint(pkId),
+      [fieldId(2, fieldTypeUint32)],
+      encodeVarint(spkId),
+      [fieldId(3, fieldTypeByteArray), ik.length],
+      ik,
+      [fieldId(4, fieldTypeByteArray), ek.length],
+      ek,
+      [fieldId(5, fieldTypeByteArray), msg.length],
+      msg,
+    ]);
+  }
+}