add DHT22

[mkinney]

Add DHT22 sensor (similar to the DHT11 sensor, but the library does different stuff):

For example, from DHT.cpp:

    switch (_type) {
    case DHT11:
    case DHT12:
      f = data[0] + data[1] * 0.1;
      break;
    case DHT22:
    case DHT21:
      f = ((word)data[0]) << 8 | data[1];
      f *= 0.1;
      break;
    }

[mkinney]

I guess I needed to merge the protobufs change before the CI build.

[mkinney]

This compiles and runs ok on my Heltec, but I cannot seem to get any readings from it. I've got some DHT11's on order. Also, going to see if Garth can send me some soldered headers.

[crossan007]

For what it's worth, my experience with the DHT11 has bee that comms is rather flaky (hence the slight overkill on managing retry options)