implement websocket based digital stage interface

[gisogrimm]

implement the digital stage interface based on typescript reference implementation #6, probably using boost websockets

[alessandrostone]

Implemented using cpprestsdk; client is able to use ds-config file to read credentials, login into digital-stage, get jwt, start websocket listener, send and receive messages, parse (some) messages, receive closing message and properly close the client.

Missing tascar integration for digital-stage( WIP )