Add support for SHACL-AF Annotation Properties (for validation results)

As mentioned in #234 SHACL-AF defines a section on https://w3c.github.io/shacl/shacl-af/#sparql-constraints-annotations

These are already in the shacl.ttl file and the description looks pretty much straight-forward. It was one of the features that were moved out of SHACL Core 1.0 due to lack of time.

They should be copied into SHACL-SPARQL.