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/verify.go

@@ -0,0 +1,7 @@
+package cmd
+
+import "github.com/spf13/cobra"
+
+func newVerifyCmd() *cobra.Command {
+	return &cobra.Command{Use: "verify ARCHIVE", Short: "Check an archive against the key in hand"}
+}