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.
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