Service desk email address for gitlab.com domain

This request is related to gitlab-org/gitlab#196827 (closed).

For service desk (https://docs.gitlab.com/ee/user/project/service_desk.html) we would like to accept incoming emails from a separate service desk specific email address - ideally contact+{key}@gitlab.com, configuration should be similar to the existing incoming+{key}@gitlab.com address.

If contact is not available/acceptable/suitable, it can be also other email address suitable for project service desk service (discussed in gitlab-org/gitlab#196827 (comment 279597710)).