Add custom data from API call to webhook response

[brcebn]

Currently, no custom data are allowed to be passed from API call that will be transferred to Webhook.

It's always be useful for the client to be able to add custom data for themselves they will catch on webhook calls.

Those custom data might be shared/transferred via the JWT or directly with another HEADER.

AWS has implemented it years ago amazon-ses-now-includes-original-message-headers-in-notifications