Feature request: spam scoring / spamassassin integration

[t-lo]

This is a good first issue to start with if you'd like to become a contributor to the mailserver project.

Add spam scoring to the mailserver, e.g via spamassassin or other spam scanners available on Alpine. The feature should be added directly to the mailserver image, i.e. no external container should be used.

The spam scorer should be integrated directly with postfix, e.g. via postfix' content_filter feature. It should provide a spam rating in email headers so users can use it in their SIEVE scripts.