[OAuth] Support QtWebEngine usage as the user-agent

During the authorization stage the user interacts with the Authorization Server. The interaction is specific to the server, and is done with a web user-agent.

The main and recommended way is to use the system browser because the user may have already signed in to a service, and when the browser is aware of that, the sequence is more convenient.

However there are still use cases to use an embedded web view (QtWebEngine in Qt's case) for this. Notably it can be integrated more seamlessly as part of the application, and also it's possible to use different redirect_uri scheme (com.mycustom.uri:// and https://) more freely, because the web view's navigation is in the application's control.

This item is about enabling the support for it, which in all likelihood requires some API additions (for convenience) and documentation how to do it. The QtNetworkAuth library itself won't get QtWebEngine dependency, but rather applications can use it and supply the necessary information to QtNetworkAuth library.