Add endpoint for user namespace callout

What does this MR do and why?

The User::NamespaceCallout was created, but the endpoint to create callouts didn't exist. We'll be using this in !95142 (merged) hence this MR.

The functionality is fairly similar to the work that was done for user group callouts in !68785 (merged) or project callouts in !94144 (merged)

Screenshots or screen recordings

No UI changes

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.