p2p/discover: add initial discovery v5 implementation (#20750)

This adds an implementation of the current discovery v5 spec.

There is full integration with cmd/devp2p and enode.Iterator in this
version. In theory we could enable the new protocol as a replacement of
discovery v4 at any time. In practice, there will likely be a few more
changes to the spec and implementation before this can happen.

p2p/discover/table.go

@@ -424,6 +424,10 @@ func (tab *Table) len() (n int) {
 // bucket returns the bucket for the given node ID hash.
 func (tab *Table) bucket(id enode.ID) *bucket {
 	d := enode.LogDist(tab.self().ID(), id)
+	return tab.bucketAtDistance(d)
+}
+
+func (tab *Table) bucketAtDistance(d int) *bucket {
 	if d <= bucketMinDistance {
 		return tab.buckets[0]
 	}