Add "auditors" parameter to users API

What does this MR do and why?

This MR creates a new optional parameter for Premium licenses which will allow the administrator user to return a list of all users who are auditors. This matches the optional parameter for administrators or those who have the same SAML provider ID.

See GitLab issue for motivation: #418023

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Create a user with the auditor capability. Execute the GET API end point with the parameters of ?auditors=true as an administrator and verify that the user is returned.

MR acceptance checklist

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