Integrate auth.AccessController into registry app

This changeset integrates the AccessController into the main registry app. This
includes support for configuration and a test implementation, called "silly"
auth. Auth is only enabled if the configuration is present but takes measure to
ensure that configuration errors don't allow the appserver to start with open
access.

api/v2/descriptors.go

@@ -39,6 +39,14 @@ var ErrorDescriptors = []ErrorDescriptor{
 		Description: `Generic error returned when the error does not have an
 		API classification.`,
 	},
+	{
+		Code:    ErrorCodeUnauthorized,
+		Value:   "UNAUTHORIZED",
+		Message: "access to the requested resource is not authorized",
+		Description: `The access controller denied access for the operation on
+		a resource. Often this will be accompanied by a 401 Unauthorized
+		response status.`,
+	},
 	{
 		Code:    ErrorCodeDigestInvalid,
 		Value:   "DIGEST_INVALID",