Remove `messenger.go` and move the interface definition to `manager`
and the `Message` struct to the `models` package, removing superflous
and redundant message structs used in multiple places.
- Add new `headers[]` column to the campain table.
- Add new headers box to the campaign UI that takes a JSON array of
custom headers like the headers on the SMTP settings UI.
- Headers are added to e-mails and messenger postback webhooks.
- Add cypress tests.
Closes#514.
This is a major feature that builds upon the `Messenger` interface
that has been in listmonk since its inception (with SMTP as the only
messenger). This commit introduces a new Messenger implementation, an
HTTP "postback", that can post campaign messages as a standard JSON
payload to arbitrary HTTP servers. These servers can in turn push them
to FCM, SMS, or any or any such upstream, enabling listmonk to be a
generic campaign messenger for any type of communication, not just
e-mails.
Postback HTTP endpoints can be defined in settings and they can be
selected on campaigns.
