feat: Add the vantagectl module and its cobra root

Its own module rather than a package under shared, so cobra and pflag stay
out of the module graphs of server, admin and sitesvc, which never use
them.

internal/cmd/backup.go

@@ -0,0 +1,7 @@
+package cmd
+
+import "github.com/spf13/cobra"
+
+func newBackupCmd() *cobra.Command {
+	return &cobra.Command{Use: "backup", Short: "Write an archive of the database"}
+}