cmd/geth: add support for sepolia testnet (#23730)

* cmd/geth: add support for sepolia testnet

* core: last details on sepolia genesis

* params: fix sepolia hash + reduce testing code

* Update params/bootnodes.go

* cmd/geth: fix attach path for sepolia

* params: update bootnodes

* params: fix

* core: fix docstring

* params: add sepolia CHT

cmd/geth/chaincmd.go

@@ -66,6 +66,7 @@ It expects the genesis file as argument.`,
 		Flags: []cli.Flag{
 			utils.MainnetFlag,
 			utils.RopstenFlag,
+			utils.SepoliaFlag,
 			utils.RinkebyFlag,
 			utils.GoerliFlag,
 		},