Discovery ENR extension implementation plan

[gumb0]

Relevant EIPs:

• https://eips.ethereum.org/EIPS/eip-778

• https://eips.ethereum.org/EIPS/eip-868

• ENR class implemenation; creating initial ENR for Host; saving/loading from network.rlp ENR record for host #5537

• ENR Server - add enr-seq field to Pings and Pongs that we send; support answering ENRRequest packets.

• Updating Host's ENR (when we get new network config from the command line and when we get external IP from the incoming packet)

• ENR Client - change our discovery algorithm to query the nodes for ENR when we see they support it

[gumb0]

ENR Client - change our discovery algorithm to query the nodes for ENR when we see they support it.

I've decided to skip this part for now, because there's not much sense in requesting ENRs in v4 discovery. v4 ENR Extension is mostly useful for the discovery v5 nodes bootstrapping from v4 nodes.

So ENR client will be part of the v5 implementation, if we ever get to it.
(And I tend to think that if we get there, it's better to implement v5 from scratch, instead of reusing anything from v4, because v4 implementation code is not great)