Eventgrid subscription

[eventsubscriptions](https://docs.microsoft.com/en-us/azure/templates/microsoft.eventgrid/eventsubscriptions?tabs=json)

### Issue Details
Given you are creating an eventgrid subscription of type Web hook, how can i set custom delivery properties (static header) on the event grid subscription in arm or bicep?

Start with the Event Grid eventsubscriptions documentation linked in the issue and inspect the webhook subscription schema for custom delivery properties. Determine how the setting is represented in ARM or Bicep, then provide the supported configuration or limitation as documentation; done means the question has a clear, actionable answer.

Written by the indexing model from the issue text.