cmd/ethkey: add command to change key passphrase (#16516)

This change introduces 

    ethkey changepassphrase <keyfile>

to change the passphrase of a key file.

cmd/ethkey/main.go

@@ -38,6 +38,7 @@ func init() {
 	app.Commands = []cli.Command{
 		commandGenerate,
 		commandInspect,
+		commandChangePassphrase,
 		commandSignMessage,
 		commandVerifyMessage,
 	}