Should provide named argument in interface `Storage`

[Eric Chang]

e.g. In CreateBucket() method, the second argument is bucket name, the third argument is region,
but it's hard to know which argument we need to provide without digging into actual implementation.

// Storage for s3 and disk
type Storage interface {
	// CreateBucket for create new folder
	CreateBucket(context.Context, string, string) error
}

So, it should be:

// Storage for s3 and disk
type Storage interface {
	// CreateBucket for create new folder
	CreateBucket(ctx context.Context, bucketName string, region string) error
}