Allow the SignalR C++ client to be built without using cpprestsdk

[analogrelay]

Epic #5301

We have customers interested in using SignalR with a different backend networking stack. We'll provide abstractions (http_client, websocket_client, etc.) and use cpprestsdk to provide default implementations but we should have a #define that can be used to strip those default implementations so that they aren't in the binary when they aren't being used.