feat: Working VoIP calling implementation (Flutter + Android)

Working video and audio calls, as well as android integration

lib/models/registration_state.dart

@@ -0,0 +1,11 @@
+enum RegistrationState {
+  None,
+  Progress,
+  Ok,
+  Cleared,
+  Failed;
+
+  static RegistrationState fromOrdinal(int ordinal) {
+    return values[ordinal];
+  }
+}