Add support for template variables in webhook authorization headers

I would like to be able to set a webhook authorization header on a template and pass that in to a webhook request. The use case here is we would like to create 1 inbox that can send webhooks to different organizations on our system. A good example is a franchise chain that has separate owners, and therefore separate permissions, but the emails across all stores are exactly the same. Currently this requires a unique webhook to be set up per franchise owner and for large franchises this will make the webhook section very overwhelming.