shell/network-agent: Add method to add VPN secrets to agent response

shell/network-agent: Add method to add VPN secrets to agent response

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>