wpa_supplicant: Add SIGNAL_MONITOR command

wpa_supplicant: Add SIGNAL_MONITOR command

SIGNAL_MONITOR THRESHOLD=DD HYSTERESIS=DD command will request signal
strength monitoring events based on there having been requested amount
of drop in the signal strength. The threshold value is the RSSI
threshold in dBm for the event to be sent. 0 threshold can be used to
disable monitoring. The hysteresis value is RSSI hysteresis in dB to
specify the minimum amount of change before a consecutive event is
reported.

With nl80211 driver interface, these values map to the
NL80211_CMD_SET_CQM command with NL80211_ATTR_CQM_RSSI_THOLD and
NL80211_ATTR_CQM_RSSI_HYST attributes to the driver.

This command cannot be used when bgscan module is in use since that
depends on being able to control the connection monitoring parameters.

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>