exim4-daemon-heavy: investigate enabling experimental ARC support

[Timothy Allen]

In their new Email Sender Guidelines <https://support.google.com/mail/answer/81126>, Google has listed a new requirement for all senders, from from February 2024:

 "If you regularly forward email, including using mailing lists or inbound gateways, add ARC headers to outgoing email. ARC headers indicate the message was forwarded and identify you as the forwarder."

This would also apply to anyone running Exim who has a user forwarding email to their Gmail account, which I suspect would describe many, if not most, Exim installations.

ARC is currently listed as experimental in Exim, under the compile-time flag EXPERIMENTAL_ARC=yes. EXPERIMENTAL_ARC has no external dependencies, but it may be a bridge too far to enable an experimental feature.

It would be extremely helpful if the next LTS exim4-daemon-heavy were able to do one of the following:

- enable EXPERIMENTAL_ARC;
- engage with upstream to see if ARC is ready to be marked as stable; or
- create an external utility that could handle ARC requirements, like spf-tools-perl.

Thank you!