Use DSSE artifactType in referrers (#95)

* bug: Use DSSE media types for artifactType

* Don't serialize DSSE extension if not present

* Update pkg/attestation/types.go

Co-authored-by: Joel Kamp <joel.kamp@docker.com>

* Don't error on no referrers

---------

Co-authored-by: Joel Kamp <joel.kamp@docker.com>

pkg/attestation/types.go

@@ -57,7 +57,7 @@ type Envelope struct {
 type Signature struct {
 	KeyID     string    `json:"keyid"`
 	Sig       string    `json:"sig"`
-	Extension Extension `json:"extension"`
+	Extension Extension `json:"extension,omitempty"`
 }
 type Extension struct {
 	Kind string              `json:"kind"`