shellDBus: Add new OpenExtensionPrefs method

shellDBus: Add new OpenExtensionPrefs method

Unlike any other methods in the Extensions API, LaunchExtensionPrefs()
opens what appears to be an application dialog, except that it is
really a separate application that the caller has no control over.

In order to address that, add a new OpenExtensionPrefs() method that
takes additional parameters (modelled after the desktop portal APIs)
that will make it possible to improve the behavior in the future.

The new parameters are ignored for now, but pushing the API out now
will allow us to fill in the functionality post the .0 release.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1074