Expose RSSI in WifiManager for the established connection

[siddacious]

It would be great to be able to do something like wifi.rssi or more descriptively wifi.signal_strength so that one could periodically log/display the signal strength.

[brentru]

Great idea - you could expose this method: https://github.com/adafruit/Adafruit_CircuitPython_ESP32SPI/blob/master/adafruit_esp32spi/adafruit_esp32spi.py#L386

[makermelissa]

That should be easy enough to do.

[makermelissa]

Fixed #15 via #16 and #18.