client: Send User-Agent header on WebSocket connection requests (#35280)

This PR makes it so we send the `User-Agent` header on the WebSocket
connection requests when connecting to Collab.

We use the user agent set on the parent HTTP client.

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -2023,6 +2023,10 @@ impl HttpClient for NullHttpClient {
         .boxed()
     }
 
+    fn user_agent(&self) -> Option<&http_client::http::HeaderValue> {
+        None
+    }
+
     fn proxy(&self) -> Option<&Url> {
         None
     }