ExJsonSchema.Validator implementation for "required" attributes.
See: https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.4.3 https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-6.17 https://tools.ietf.org/html/draft-handrews-json-schema-validation-01#section-6.5.3