Fix #1027 - Support templating mail

[bwbroersma]

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Proposed changes

• Fix Support templating mail #1027
  Added functions to keep it DRY

Checklist

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and/or modules/README.md)

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[bwbroersma]

I have hereby read the F5 CLA and agree to its terms

[thresheek]

Thank you for your contribution! It does make sense to have it templated.

I've started work to add stream templates to the tests to make sure those work as intended: https://github.com/thresheek/docker-nginx/commits/tests-templates/

It would make sense to test the mail template functionality as well.

CI issues seem to be unrelated, opened #1029 to fix those.

[bwbroersma]

@thresheek Hey, sorry for sending this PR a bit early, I did some work on it in October last year, and lost track of progress. Since this is a dependency for another PR and forgetting it wasn't finished I created this pull request, later realized it was still missing tests (and some renaming variables were not yet committed, and almost got lost in the rebase).

A simple test would be:

# smtp.conf.mail-template
server_name server-name;
auth_http http://127.0.0.1:9000/;
server {
    listen 25;
    protocol smtp;
}

Optional auth handler:

# smtp_auth_http.conf.template
server {
    listen 127.0.0.1:9000;
    location / {
        default_type text/plain;
        add_header Auth-Status "Login not supported since this is a dummy nginx smtp handler";
        add_header Auth-Error-Code "550 5.3.5";
        return 200;
    }
}

A test could be:

$ echo "QUIT\r\n" | nc 127.0.0.1 25

which should results in:

220 server-name ESMTP ready
221 2.0.0 Bye