Add mail_room logfile config

• Add structured logging file name
• Attempt at test

Following: https://docs.gitlab.com/omnibus/settings/gitlab.yml.html#adding-a-new-setting-to-gitlabyml

as part of this MR: gitlab!19186 (merged)

fluentd MR: gitlab-cookbooks/gitlab_fluentd!96 (merged)

Related to gitlab#35108 (closed)