Support templating mail

[bwbroersma]

Feature Overview

Support templating for mail block, similar to stream:

• Add an entrypoint for configuring and templating streams #782

Alternatives Considered

I first considered adding 40-add-mail-include.sh:

#!/bin/sh

set -eu

LC_ALL=C
ME=$(basename "$0")
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

touch /etc/nginx/nginx.conf 2>/dev/null || { echo >&2 "$ME: error: can not modify /etc/nginx/nginx.conf (read-only file system?)"; exit 0; }

sed -i -r -z 's@(\}\n)$@\1# Added by '"$ME"' on '"$(date)"'\nmail {\n    include conf.d/*.mail-conf;\n}\n@' /etc/nginx/nginx.conf

but in the review it was noted upstream support, similar to stream, would be a better solution.

Additional Context

No response