Add switch for email notifications to code owners

Problem to solve

Currently when a merge request is made and if a CODEOWNERS is specified in the repository, the CODEOWNERS that will approve the MR are not notified via email. This was intentionally disabled in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8466 so that code owners were not spammed.

A proposed solution might be to include a switch at the global/project level to allow users to enable/disable email notifications to code owners, with the notification disabled by default (as it is now).

Intended users

Further details

Proposal

Notify code owners and approvers when the merge request is created, or when they become an eligible approver as more commits are pushed.

Until a user is a participant they should not receive every notification.

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references