[RFE] Support of captive portal API

RFC 8910 defines DHCPv4 and DHCPv6 options to detect captive portal URL for the connected network. RFC 8908 defines API to obtain captive portal status in automated way.

I think captive portal detection is done for any connection anyway, so it would be best if the HTTPS could be used for it straight away. Instead of catching HTTP redirects to known names. A minimum support should be requesting URL in DHCP request and exposing it to possible other services, which may display detailed information about the network or even provide way to auto-login to such network.

Tested at least NetworkManager-1.42.0-1.fc38.x86_64 does not request RFC 8910 Captive-Portal option.

Ideally captive portal information should be visible in NM applets if provided by the network.