Add `namespace` to the list of optional Feature API param

What does this MR do and why?

Related to #353117 (closed)

In !79555 (merged) we added support for namespace actor param for features API endpoint but we didn't declare the param for the endpoint. The endpoint works anyway since we access the param directly but for consistency we should add it as optional and mutually_exclusive param.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

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.