make a DMARC plugin so we don't have to manually fix new mailman3 mailing lists

in #40471 (closed), we focused on upgrading the mailman server from legacy, which we did. in doing so, we kept some of the warts that come with MM3. one of those is that it does not implement correct DMARC mitigations by default.

we have a mitigation script documented, but it would be much better to have a "style" that would just do the right thing. i've been trying to figure out how exactly to write that plugin, but so far have failed to come up with the exact, magic formula. i'm hoping that upstream can shed a light on this.

an alternative is to just patch the debian package to fix the defaults, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1086707 for that discussion (or, as of 2024-11-05, lack thereof).

for now, the workaround remains.