eth/catalyst: add catalyst API prototype (#22641)

This change adds the --catalyst flag, enabling an RPC API for eth2 integration.
In this initial version, catalyst mode also disables all peer-to-peer networking.

Co-authored-by: Mikhail Kalinin <noblesse.knight@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>

cmd/geth/main.go

@@ -151,6 +151,7 @@ var (
 		utils.EVMInterpreterFlag,
 		utils.MinerNotifyFullFlag,
 		configFileFlag,
+		utils.CatalystFlag,
 	}
 
 	rpcFlags = []cli.Flag{