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/enode/nodedb_test.go

@@ -462,3 +462,14 @@ func TestDBExpiration(t *testing.T) {
 		}
 	}
 }
+
+// This test checks that expiration works when discovery v5 data is present
+// in the database.
+func TestDBExpireV5(t *testing.T) {
+	db, _ := OpenDB("")
+	defer db.Close()
+
+	ip := net.IP{127, 0, 0, 1}
+	db.UpdateFindFailsV5(ID{}, ip, 4)
+	db.expireNodes()
+}