Add ADSP TXT record #188
Description
There is a "historic" RFC https://datatracker.ietf.org/doc/html/rfc5617
It is declared "historic" because of no widespread deployment: https://datatracker.ietf.org/doc/status-change-adsp-rfc5617-to-historic/
However, it is declared as supported by at least fastmail's https://github.com/fastmail/authentication_milter
OpenDKIM has a release note from 2014-12-27 saying "Discontinue support for ADSP", only unused artifacts like ADSP_DISCARDABLE_TEXT in opendkim.h remain in the code.
Anyway, it should not hurt to publish a TXT record _adsp._domainkey.example.org saying dkim=discardable. This is the DKIM policy that we actually want and implement in #186.