Add functionality to handle frame with custom opcodes

[xanderdunn]

My WebSocket client is frequently receiving frames with unknown frame opcodes: [oatpp::web::protocol::websocket::AsyncWebSocket::handleFrameAsync(){HandleFrameCoroutine}]: Unknown frame. I would like to be able to inspect these frame opcodes to know what the opcodes are and potentially respond to them.

The suggested solution of creating a custom implementation of listenAsync is not currently possible because it accesses private members such as readFrameHeaderAsync.