networkAgent: Handle 'vpn' connections when delaying request

networkAgent: Handle 'vpn' connections when delaying request

Since commit 90a08ba0b6a, we only open a network secret dialog immediately
in response to user action, and show a notification otherwise.

While for the actual request VPNs are handled separately from other connections,
this isn't true when we show the notification - we need to handle 'vpn' together
with the other types there, or we fall through to the default 'invalid type'
exception.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2008