feat: Add TLS client certificate options to the mailer connection

[rhclayto]

Pre-submission checklist

• I have searched for existing open or closed issue reports with the same feature request.

Description

The underlying go-mail library is able to do TLS client certificates for mutual TLS. It would be good to have this option. A pull request is incoming for review.

Which alternatives did you consider using instead?

No response

[rhclayto]

PR: #2322