[Outgoing Webhooks] Allow user to edit the request headers

[I-Dont-Remember]

There are plenty of cases where a request will need an Authorization header - and currently people are dead in the water with our Webhooks.

For example, trying to create records using the Airtable API, you'll need to pass:

curl -X POST https://api.airtable.com/v0/3243554543/Table%201 \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  --data '{
  "records": [
    {
      "fields": {}
    },
    {
      "fields": {}
    }
  ]
}'