What does this MR do?

This adds JWT authentication to support delivering e-mail via an API handler.

Diff: https://my.diffend.io/gems/gitlab-mail_room/0.0.15/0.0.18

Related issues

https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1457

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
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated