drivers: wifi: esp_at: support WiFi 'iface_status' callback

This allows to fetch information about currently connected WiFi network,
e.g. with WiFi mgmt shell commands:

  uart:~$ wifi status
  Status: successful
  ==================
  State: COMPLETED
  Interface Mode: STATION
  Link Mode: UNKNOWN
  SSID: Tp-Link
  BSSID: xx:xx:xx:xx:xx:xx
  Band: 2.4GHz
  Channel: 13
  Security: UNKNOWN
  MFP: UNKNOWN
  RSSI: -39

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>