WebSocketize the POST /v1/download/image endpoint

[programarivm]

A new service needs to be created on port 7443 to allow the transmission of binary data. More specifically, the /transmit command will be used as a counterpart of the POST /v1/download/image endpoint to transmit chessboard images based on a FEN string.

See:

• Transmission of Binary Data in WebSocket Protocol
• chesslablab/chess-api@9b4b83f
• walkor/workerman#235
• chesslablab/js-utils#45