Use MatchSubjctMappings in subject mapping builtin instead of passing in attributes

[elizabethhealy]

Currently we call GetAttributeValuesByFqns to get subject mappings for the specified scope in GetEntitlements. These are then passed through the rego to the subject mapping builtin where the mappings are actually evaluated.

Instead call MatchSubjctMappings in the builtin to retrieve the relevant subject mappings using the entity representations returned from ERS.