Add Microsoft Graph config support for MailRoom

This commit adds support for Microsoft Graph API with OAuth2 to MailRoom: https://github.com/tpitale/mail_room/#microsoft-graph-configuration

This needs gitlab-org/build/CNG!639 (merged) for Microsoft Graph to work. IMAP will work without this.

Related issues:

1. gitlab-org/gitlab#214900 (closed)
2. https://github.com/tpitale/mail_room/pull/125

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