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/usage.go

@@ -235,6 +235,7 @@ var AppHelpFlagGroups = []flags.FlagGroup{
 			utils.SnapshotFlag,
 			utils.BloomFilterSizeFlag,
 			cli.HelpFlag,
+			utils.CatalystFlag,
 		},
 	},
 }