stigs: new sketch

This is NOT ready for merging, just for review.

@zzamboni @jborozan please review and we can discuss.  This is namespaced under `cfdc_stigs`.

@jborozan I need your full contact info for sketch authorship, will be shown together with the other CFEngine people who worked on it, if you're OK with that.  Also note the license for your contributions is MIT.