chore: Return webhook version in the header (#26139)

* init

* add tests

* fix type

* type fix

* fix

* fix tests

* fix test

docs/developing/guides/automation/webhooks.mdx

@@ -59,6 +59,11 @@ All webhook payloads are wrapped in the following structure:
 }
 ```
 
+<Note>
+Webhook payloads are versioned. The version of the payload sent is included in the x-cal-webhook-version HTTP header.
+Example: x-cal-webhook-version: 2021-10-20
+</Note>
+
 Select a version and trigger event to view the example payload:
 
 <Tabs>