feat(internal event): track feature_enabled_by_namespace_ids

What does this merge request do and why?

This MR follows up the following point for gitlab.com tracking:

This header value should be extracted in AI Gateway and set to feature_enabled_by_namespace_ids in standard context

#491 (comment 2015655480).

Later, we will extend the clients in GitLab-Rails to send X-Gitlab-Feature-Enabled-By-Namespace-Ids HTTP header to AI Gateway.

Related to feat(internal event): track feature_enabled_by_... (!1144 - merged)

How to set up and validate locally

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

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.