Add structured logging for mail_room
https://github.com/tpitale/mail_room/issues/76 is an upstream issue for adding structured logging. We already use structured logging in GitLab itself, so this is a good fit and would give us better visibility.
It's also important for infrastructure: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7204
We have had PRs accepted in the past (many of https://github.com/tpitale/mail_room/pulls?q=is%3Apr+is%3Aclosed are from GitLab team members), so this should be OK to contribute.