[Feature Request] Add support for universal tags for Opsgenie

[gjethwani]

Hi,

We use Terraform to manage our detectors which notify us using Opsgenie. For routing purposes, we would like to add universal tags to all detectors. Something that designates what service this alert is coming from. Can we add support for these tags in the opsgenie_notification string?

I'm happy to contribute this change, but can you confirm what i would have to add to this object to support tags being added to the Opsgenie notification?

case OpsgenieNotificationType:
	n = &notification.OpsgenieNotification{
		Type:          vars[0],
		CredentialId:  vars[1],
		ResponderName: vars[2],
		ResponderId:   vars[3],
		ResponderType: vars[4],
	}