Support Proxy Configuration #40
Description
When attempting to connect to Salesforce using the CometD.NetCore library from within an organization's network, which is behind a proxy, the connection fails. This is because the library uses HttpWebRequest internally, which does not automatically inherit the proxy settings configured in HttpClient.
To support proxy configurations, it would be helpful if the LongPollingTransport and other relevant classes could accept proxy settings. This would involve modifying the classes to use an HttpClient that is configured with the necessary proxy settings or directly setting the proxy on HttpWebRequest.