[#1402] cli: Add cid flag constant

Also, move common flag constants in `root` file of the `storagegroup`
package.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

cmd/neofs-cli/modules/storagegroup/root.go

@@ -19,6 +19,12 @@ var Cmd = &cobra.Command{
 	},
 }
 
+const (
+	sgIDFlag  = "id"
+	sgRawFlag = "raw"
+	cidFlag   = "cid"
+)
+
 func init() {
 	storageGroupChildCommands := []*cobra.Command{
 		sgPutCmd,