Configure OpenDKIM

Summary:
This configuration enables signing + verify modes,
and refer to the keys and signing tables.

References:

• man 8 opendkim
• man 5 opendkim.conf

Ref T2066.

Test Plan:

• Deploy to Hervil
• Start the service
• Check socket path and ownership

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2066

Differential Revision: https://devcentral.nasqueron.org/D3552