Implement parsing of certificate validity duration and renewal grace period from service labels

[simu]

Currently, both the certificate validity duration and the renewal grace period are hard-coded in the controller. We should implement optional service labels which users can set to customize the validity duration and renewal grace period for their service certificates.

Current implementation:

k8s-service-ca-controller/certs/create.go

Lines 113 to 129 in f2e105f

	func certDurationFromSvc(svc *corev1.Service) (*metav1.Duration, error) {
	// TODO: annotation/label on svc
	d, err := time.ParseDuration("2160h")
	if err != nil {
	return nil, err
	}
	return &metav1.Duration{Duration: d}, nil
	}
	func certRenewBeforeFromSvc(svc *corev1.Service) (*metav1.Duration, error) {
	// TODO: annotation/label on svc
	d, err := time.ParseDuration("360h")
	if err != nil {
	return nil, err
	}
	return &metav1.Duration{Duration: d}, nil
	}