core/forkid: implement the forkid EIP, announce via ENR (#19738)

* eth: chain config (genesis + fork) ENR entry

* core/forkid, eth: protocol independent fork ID, update to CRC32 spec

* core/forkid, eth: make forkid a struct, next uint64, enr struct, RLP

* core/forkid: change forkhash rlp encoding from int to [4]byte

* eth: fixup eth entry a bit and update it every block

* eth: fix lint

* eth: fix crash in ethclient tests

p2p/server.go

@@ -286,6 +286,11 @@ func (c *conn) set(f connFlag, val bool) {
 	}
 }
 
+// LocalNode returns the local node record.
+func (srv *Server) LocalNode() *enode.LocalNode {
+	return srv.localnode
+}
+
 // Peers returns all connected peers.
 func (srv *Server) Peers() []*Peer {
 	var ps []*Peer