Allow passive mdns discovery #3401
Description
We should allow configuring the MdnsDiscovery so that it only listens, not publishes.
Just like with other discovery mechanisms, a short lived ephemeral node needs to be able to listen to mdns packets without having to broadcast their own random and short lived node id.
This is mainly useful for a client/server protocol on top of a p2p connection, but I find that I write those often. E.g. you want to look up a printer, you don't need to broadcast your own node id and spam the local network.