feat: allow resource path to be set in connection URI (socketio#134)

Please note that this implementation differs from the JS one, where the
resource path is the namespace you want to reach:

```js
const socket = io("https://example.com/my-namespace", {
  path: "/my-custom-path/"
});
```

Here, we will have:

```C++
sio::client h;
h.connect("https://example.com/my-custom-path/");
h.socket("/my-namespace")->emit("hello");
```

Author: joelnordell

src/internal/sio_client_impl.cpp

@@ -238,7 +238,12 @@ namespace sio
             } else {
                 ss<<uo.get_host();
             }
-            ss<<":"<<uo.get_port()<<"/socket.io/?EIO=4&transport=websocket";
+
+            // If a resource path was included in the URI, use that, otherwise
+            // use the default /socket.io/.
+            const std::string path(uo.get_resource() == "/" ? "/socket.io/" : uo.get_resource());
+
+            ss<<":"<<uo.get_port()<<path<<"?EIO=4&transport=websocket";
             if(m_sid.size()>0){
                 ss<<"&sid="<<m_sid;
             }