fix: Multiple network flags should prevent the BN to start (#14169)

* Implement Initial Logic

* Include check in main.go

* Add tests for multiple flags

* remove usage of append

* remove config/features dependency

* Move ValidateNetworkFlags to config/features

* Nit

* removed NetworkFlags from cmd

* remove usage of empty string literal

* add comment

* add flag validation to prysctl validator-exit

---------

Co-authored-by: Manu NALEPA <enalepa@offchainlabs.com>

cmd/prysmctl/validator/cmd.go

@@ -175,6 +175,9 @@ var Commands = []*cli.Command{
 					if err := cmd.LoadFlagsFromConfig(cliCtx, cliCtx.Command.Flags); err != nil {
 						return err
 					}
+					if err := features.ValidateNetworkFlags(cliCtx); err != nil {
+						return err
+					}
 					if err := tos.VerifyTosAcceptedOrPrompt(cliCtx); err != nil {
 						return err
 					}