Upgrade to latest CEL specification

CEL specification update:

[ ] Compare CEL specification when this was first released with current CEL specification to get the list of changes.
[ ] Update documentation with very visible details on specific CEL specification used.
[ ] Rewrite and expand test cases to cover latest CEL specification.
[ ] Prioritize rework to bring this project into compliance. Create individual issues and a parent tracking issue.

cloud-custodian / cel-python