Make referrers attestations OCI compliant (#80)

* Single attestation when creating VSA

* Create single layer images for referrers attestations

* Move mock to test package. Add artifacts test

* Add test for envelope detection

* Add tests for image/index saving

* Add mirror tests

* Remove AttestationImage field from AttestationManifest

* Update naming. strictReferers != laxReferrers

* Add specific test for SaveReferrers

pkg/policy/policy_test.go

@@ -38,7 +38,7 @@ func TestRegoEvaluator_Evaluate(t *testing.T) {
 
 	re := policy.NewRegoEvaluator(true)
 
-	defaultResolver := oci.MockResolver{
+	defaultResolver := test.MockResolver{
 		Envs: []*attestation.Envelope{loadAttestation(t, ExampleAttestation)},
 	}