[SignalR-Client-Cpp] Support for LongPolling Transport

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I am using SignalR-Client-Cpp from inside a corporate network to connect to a public (online) signalR hub. The connection goes through a WAF which is incompatible to websockets.

### Describe the solution you'd like

Automatic fallback to use Long Polling [Transport](https://github.com/aspnet/SignalR/blob/release/2.1/specs/TransportProtocols.md) when the `Upgrade` to Websockets is not possible.

```log
[error ] [websocket transport] exception when connecting to the server: set_fail_handler: 20: Invalid HTTP status.
[error ] transport could not connect due to: set_fail_handler: 20: Invalid HTTP status.
[info ] stopping connection
[info ] acquired lock in shutdown()
set_fail_handler: 20: Invalid HTTP status.
```

### Additional context

_No response_