Remember endpoint we authenticated against in returned SyncAuth

[sartak]

Hi, thanks for Anki. I have 1.1mil reviews going back 14 years. :)

I'm working on reusing the Rust backend in a project, and I hit a bug related to using a custom sync server.

let auth = backend.sync_login(SyncLoginRequest {
  username,
  password,
  endpoint: Some("http://custom-sync-endpoint"),
})?;

let status = backend.sync_status(auth)?;

This pair of method calls authenticates against http://custom-sync-endpoint, but then when it checks the sync status, it unexpectedly queries the public ankiweb. This is because the anki_proto::sync::SyncAuth returned in sync_login_inner() hardcodes endpoint: None.

With this PR, the SyncAuth is populated correctly so that sync_status method correctly queries http://custom-sync-endpoint instead of ankiweb. (And if the user isn't using a custom sync server, we still preserve the endpoint: None behavior.)

This bug doesn't affect desktop Anki because the Python client stashes the hkey out of the SyncAuth return value here in sync_login, and then constructs a new SyncAuth from scratch later in sync_auth.

[dae]

Thanks Shawn!