Add automatic mode negotiation

[andrew153d]

This PR adds auto negotiation. On boot, each side sends a discovery message and starts listening. The next program to run sends a discovery message that the first one will receive. Based on the boot order, each side will take a different mode.

[LollosoSi]

Assuming I understood everything in this PR, here are a couple remarks you might find useful:

1. More than two endpoints in the same channel means a reduction of channel capacity.
   • Moving to a different channel is desirable.
2. Who is primary and secondary can influence the performance of the link (found during my testing in rf24tunlink)
   Some factors I noted:
   • Quality of the radio modules
   • Execution speed of the endpoints
   • Which of the two devices streams more data

You might want to implement a similar behavior:

1. The endpoints connect to a shared channel and do their discovery/handshake
2. The primary radio scans the other channels and finds a clear one (you could do this through: RF24::testCarrier)
   2a. If no channel is available, then listen one of them and find a free address.
3. The last exchange of the handshake includes the channel and an address/id calculated in a way that could avoid accidental collisions.

Just my opinion. Cool addition