Callback method to get the response message for the handshake message.
When AuthenticatedDuplexOutputChannel receives the handshake message it calls this callback to get
the response message for the handshake message.
The response handshake message is then sent to AuthenticatedDuplexInputChannel which will
then authenticate the connection.
Returns response for the handshake message.
Returned response message must be String or byte.
If it returns null then it means the handshake message is not accepted and the connection will be closed.
channelId - service address
responseReceiverId - unique unique id representing the connection with the client
handshakeMessage - handshake message received from the service
handshake response message (must be String or byte)