Add AT+CSQ to network module

[somervda]

As part of my IOT project for a mobile vehicle (RV and Boat) I would like to report back on the cellular signal strength , I couldn't see a function for this in the network library. I made my own at the user level (See hacked version below with no error logic, but works when it works). but would be nicer to see this as a proper method in the network class.

# see https://sixfab.com/wp-content/uploads/2023/05/Quectel_BG95BG77BG600L_Series_AT_Commands_Manual_V2.0.pdf
command = "AT+CSQ"
result = picoLTE.atcom.send_at_comm(command)
# ugly bit
rssi= result['response'][0].split(":")[1].split(',')[0]
print("rssi:",(int(rssi.strip())*2)-109,"db")