The earlier logic for copying existing passwords from the DB to
SMTP and messenger settings when the frontend sent an empty
password was based on array positions (and flawed). This commit
introduces an internal UUID field to SMTP and Messenger settings
to keep track of password changes in the settings UI.
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.
Kailash Nadh