[Feature Request]: enable setting more/custom headers in Mail transform

### What would you like to happen?

It would be nice to be able to **set more/custom headers** when sending an email via "Mail" transform.

Especially we have requirements to set the following headers (examples for such custom headers):

* "Sender:" header according to RFC 5322 (in addition to the already present header "From: " which can be set via "Sender name" and "Sender address" configuration - at least as far as I understand)

* "Disposition-Notification-To" according to RFC 8098

* "Return-Receipt-To"

(Any workarounds/documentation for setting such headers with other meanse are also very welcome.)

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui, Component: Transforms

Start with the Hop GUI Mail transform and inspect how the existing Sender name and Sender address settings are handled. Determine how custom headers such as Sender, Disposition-Notification-To, and Return-Receipt-To could be configured and passed into outgoing mail; done means the requested headers can be set when sending an email.

Written by the indexing model from the issue text.