Add support for DFS (dynamic frequency selection) on supported devices.

This was a much used (and loved) feature on our Airport Extreme due to the network congestion in the area where we live (lots of access points; lots of interference). Our Airport almost always landed within the DFS frequencies to avoid interference, and as a result, generally had consistent speeds/latency as a result.

While we could install a 3rd Eero to overpower the other access points in our area, considering we have a smaller house, I believe that adding DFS into the mix would solve our problems of occasional performance degradation.

Thanks!