net: wifi: Allow to use offloaded wifi_mgmt API with native stack

wifi drivers that depends on native ethernet stack cannot perform
wifi API calls missing availability. This changes adds the ethernet_api
interface in wifi_mgmt so that it becomes possible.

Naming "offload" in "struct net_wifi_mgmt_offload" is kept because
Zephyr still has no supplicant to handle a full non-offloaded driver.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>