Add provenance pull flow for official images

Add support for pulling signed images from a version 2 registry.
Only official images within the library namespace will be pull from the
new registry and check the build signature.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

docs/types.go

@@ -32,6 +32,15 @@ type RegistryInfo struct {
 	Standalone bool   `json:"standalone"`
 }
 
+type ManifestData struct {
+	Name          string   `json:"name"`
+	Tag           string   `json:"tag"`
+	Architecture  string   `json:"architecture"`
+	BlobSums      []string `json:"blobSums"`
+	History       []string `json:"history"`
+	SchemaVersion int      `json:"schemaVersion"`
+}
+
 type APIVersion int
 
 func (av APIVersion) String() string {
@@ -45,7 +54,6 @@ var apiVersions = map[APIVersion]string{
 }
 
 const (
-	_           = iota
-	APIVersion1 = iota
+	APIVersion1 = iota + 1
 	APIVersion2
 )