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

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.