This PR creates a new /gateway:type:arm transport.
It depends on CJSON
The arm transport is not a transport by itself but is responsible
for getting the websocket endpoint from a configuration url derived from
the configured gateway host in the rdpw file or cmdline
* factor out most websocket specific code parts into websocket.c
* create wst.c (Websocket Transport) as gateway transport implementation
* introduce GatewayUrl setting that holds the websocket url
* introduce GatewayHttpExtAuthBearer that holds the HTTP Bearer
* GatewayHttpExtAuthBearer can be used by both rdg and wst
