Allow to add a new domain for OpenDKIM

Summary:
The procedure noted on Agora can actually be a new add-dkim-domain command.

Reference: https://agora.nasqueron.org/Operations_grimoire/Mail/DKIM

Ref T2066

Test Plan:
New DKIM keys have been created on Hervil with:

• add-dkim-domain ook.space
• add-dkim-domain nasqueron.org
• …

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2066

Differential Revision: https://devcentral.nasqueron.org/D3551