Add headers to the long polling transport

[vaspadi]

I’m using LongPollingTransport and needed to add a User-Agent header to the request. However, setHeaderValue had no effect in this case.

I came across PR #114 (Thanks to the author for the idea and initial implementation), which fixed a similar issue for WebSocketTransport. I used that PR as a foundation — I copied the relevant parts and adapted them to work with LongPollingTransport, adding my own changes where necessary.

Here’s how I configure the connection:

final serverUrl =
    '${hostUrl}signalr/hubs/AccountEventsHub?clientID=$clientID';
final headers = MessageHeaders();
headers.setHeaderValue(
  'User-Agent',
  httpClient.config.headers?['User-Agent'],
);

final httpOptions = HttpConnectionOptions(
  transport: HttpTransportType.LongPolling,
  headers: headers,
);

final hubConnection = HubConnectionBuilder()
    .withUrl(serverUrl, options: httpOptions)
    .withAutomaticReconnect()
    .build();
...