wifi_mgmt: Add new API for Wi-Fi status

A new net_mgmt command and event are added for interface status,
depending on the implementation the status can be returned when polled
or an unsolicited event can be send by driver whenever there is a change
in status.

This is planned to be implemented only by upcoming wpa_supplicant,
offload implementation is left for driver developers.

Signed-off-by: Krishna T <krishna.t@nordicsemi.no>