p2p: move rlpx into separate package (#21464)

This change moves the RLPx protocol implementation into a separate package,
p2p/rlpx. The new package can be used to establish RLPx connections for
protocol testing purposes.

Co-authored-by: Felix Lange <fjl@twurst.com>

cmd/devp2p/main.go

@@ -63,6 +63,7 @@ func init() {
 		discv5Command,
 		dnsCommand,
 		nodesetCommand,
+		rlpxCommand,
 	}
 }