Replace manual specification of Authorization header with SignalR-provided provider property

The property used here is listed in SignalR documentation:

	https://learn.microsoft.com/en-us/aspnet/core/signalr/authn-and-authz?view=aspnetcore-10.0#bearer-token-authentication

While in practice this probably has zero bearing on anything,
theoretically the way proposed in this commit is more correct.
As the documentation states, with some transports the token may need to
be renewed if it expires, which providing it via a header as done
previously would not achieve, while going through `API.AccessToken`
every time will perform a token refresh if one is needed.

osu.Game/Online/HubClientConnector.cs

@@ -65,7 +65,7 @@ namespace osu.Game.Online
                             options.Proxy.Credentials = CredentialCache.DefaultCredentials;
                     }
 
-                    options.Headers.Add(@"Authorization", @$"Bearer {API.AccessToken}");
+                    options.AccessTokenProvider = () => Task.FromResult<string?>(API.AccessToken);
                     // non-standard header name kept for backwards compatibility, can be removed after server side has migrated to `VERSION_HASH_HEADER`
                     options.Headers.Add(@"OsuVersionHash", versionHash);
                     options.Headers.Add(VERSION_HASH_HEADER, versionHash);