fix(authorization): remove access pdp internal AttributeInstance type and use policy proto generated struct types instead (#485)

1. removes `AttributeInstance` type
2. DRY improvements to tests in pdp_test.go
3. adds tests for existing and new helper functions (grouping, FQN
relations, etc)
4. fixes commented structure vs real structure of `EntityEntitlements`
proto and clarifies field name

Addresses #469 

Original PR #471 and author
@jakedoublev

Author: pflynn-virtru

docs/grpc/index.html

@@ -55,7 +55,7 @@ func authorizationExamples(examplesConfig *ExampleConfig) error {
 		Actions:      actions,
 		EntityChains: entityChains,
 		ResourceAttributes: []*authorization.ResourceAttribute{
-			{AttributeFqns: []string{tradeSecretAttributeValueFqn, openAttributeValueFqn}},
+			{AttributeValueFqns: []string{tradeSecretAttributeValueFqn, openAttributeValueFqn}},
 		},
 	})