fix: use madmin.Credentials for gateway interface (#12493)

the main reason is to de-couple the project from
depending on MinIO's internal/auth package, other
changes will subsequently follow.

cmd/gateway-interface.go

@@ -17,9 +17,7 @@
 
 package cmd
 
-import (
-	"github.com/minio/minio/internal/auth"
-)
+import "github.com/minio/madmin-go"
 
 // GatewayMinioSysTmp prefix is used in Azure/GCS gateway for save metadata sent by Initialize Multipart Upload API.
 const (
@@ -37,8 +35,5 @@ type Gateway interface {
 	Name() string
 
 	// NewGatewayLayer returns a new  ObjectLayer.
-	NewGatewayLayer(creds auth.Credentials) (ObjectLayer, error)
-
-	// Returns true if gateway is ready for production.
-	Production() bool
+	NewGatewayLayer(creds madmin.Credentials) (ObjectLayer, error)
 }