deploy postfix-based incoming DKIM verification on polyanthum

as part of #40581 (closed), we deployed a new transport to deliver mails directly to the bridges mail server from our postfix server. but in doing so, we dropped DKIM verification, which was piped in by procmail.

i don't want to bring procmail back, but we should restore DKIM functionality here. and this would serve as a good test bed for the broader "plan for email" (#40363 (closed), %improve mail services)

@meskio, could you let me know when the test setup is expected to enter production, to get an idea of the timeline here? thanks!