Support DKIM

[aurelien-baudet]

• JavaMail: https://github.com/markenwerk/java-utils-mail-dkim
• SendGrid: automatic (can be configured manually ?)
• Mailjet: https://app.mailjet.com/support/que-signifie-dkim-domain-keys-identified-mail,7.htm

[aurelien-baudet]

The idea is to be able to configure a DKIM provider through configuration.
The developer should not have to handle this in the code.
The original message is signed after being processed as usual.

The DKIM provider can be anything such as:

• a local certificate provider (could even create a new one on the fly)
• a remote certificate provider (could load the certificate from http server for example or a vault)

We also need to ensure that the DNS resource record is correctly setup.