API: enable user search by any user-associated email for admins

Problem to solve

GitLab usernames are free-form. As such, they can be anything that the user sets them up to be. Currently, there is no way to query the admin API to identify a GitLab user given an email, if that email is not set as a primary email. This makes it challenging to create any automation workflows where the username of a user is not known (but the email is, and that email is not set as primary email). This search functionality already exists in the admin page, so it would make sense to enable it in the API as well.

Proposal

Expand "search users by email" functionality in the admin API to include all email addresses for that user, not just the primary email.

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

Expand "search users by email" functionality in the admin API to include all email addresses for that user, not just the primary email, which would return information similar to that returned by /users/:id or /users?search=$email