Add support for discv5 discovery

[arnetheduck]

discv4 is on its way out - following the next hard fork, discv5 should be enabled by most, if not all, execution clients.

Both discv4 and discv5 should run on the same UDP port (it's possible to detect from the handshake which one is intended) - if discv4 is tricky to support, we can simply remove it.

[tersec]

While in theory it supports an option to check for discv5:

nimbus-eth1/nimbus/config.nim

Lines 125 to 128 in d2d6d86

	DiscoveryType* {.pure.} = enum
	None
	V4
	V5

and

nimbus-eth1/nimbus/config.nim

Lines 339 to 347 in d2d6d86

	discovery* {.
	desc: "Specify method to find suitable peer in an Ethereum network (None, V4, V5)"
	longDesc:
	"- None: Disables the peer discovery mechanism (manual peer addition)\n" &
	"- V4 : Node Discovery Protocol v4(default)\n" &
	"- V5 : Node Discovery Protocol v5"
	defaultValue: DiscoveryType.V4
	defaultValueDesc: $DiscoveryType.V4
	name: "discovery" .}: DiscoveryType

I'm not sure if it ever checks for which type to use:

nimbus-eth1/nimbus/nimbus_execution_client.nim

Lines 126 to 130 in d2d6d86

	# Start Eth node
	if conf.maxPeers > 0:
	nimbus.networkLoop = nimbus.ethNode.connectToNetwork(
	enableDiscovery = conf.discovery != DiscoveryType.None,
	waitForPeers = true)

[kdeme]

I'm not sure if it ever checks for which type to use:

It does not. Unused config setting.