087bd863b3
While most secrets are serialized as individual settings with a string value, all VPN secrets are serialized together as a string dict which is the value of a single setting. Incorrect serialization causes VPN secrets to not be remembered by NetworkManager. This commit adds a new method that allows adding secrets as VPN secrets specifically such that they can be correctly serialized. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1535>