content: Add InfoReaderProvider

The interface that combines both content.InfoProvider and
content.Provider was duplicated in multiple places - create one directly
in `content` package and use it instead.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

content/content.go

@@ -108,6 +108,12 @@ type Status struct {
 // WalkFunc defines the callback for a blob walk.
 type WalkFunc func(Info) error
 
+// InfoReaderProvider provides both info and reader for the specific content.
+type InfoReaderProvider interface {
+	InfoProvider
+	Provider
+}
+
 // InfoProvider provides info for content inspection.
 type InfoProvider interface {
 	// Info will return metadata about content available in the content store.