Add microsoft_graph_mailer settings

What does this MR do?

Adds a configuration setting to enable Microsoft Graph Mailer for sending emails.

Related to gitlab-org&8259 (closed), gitlab-org/gitlab#365524 (closed)

Related MR: gitlab-org/gitlab!97372 (merged), gitlab-org/omnibus-gitlab!6369 (merged)

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
  • gitlab-org/omnibus-gitlab!6369 (merged)