RFC: New self keyword

[johanfylling]

We're adding a new keyword to Rego: self. The first iteration of which will grant rules access to annotations in the metadata blocks associated with the rule in question.

Please have a look at this document, outlining the self keyword and how it's going to work. Don't hesitate to comment in the doc, in this discussion or on slack.

Also, we are currently expanding the set of supported annotations; look out for those in the coming releases. Ideas for useful annotations are also welcome!