Feature: Discovery in Parachains

[rvalle]

I am testing in system parachains, and authorities (collators) don't seem to be discoverable in the same way.

System parachains use Aura instead of Babe.

The list of authorities can be obtained from aura.authorities, however there seem to be no DHT record as we see in the rely chain.

Note that the aura authority keys have a different type (aura) instead of (audi), which stands for Authority Discovery.