Track gitlab saas namespace ids in snowplow event

What does this merge request do and why?

This MR adds gitlab_saas_namespace_ids to the snowplow instrument, which is requested by https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/340+.

This is similar to https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/277+.

GitLab-Rails counter part: Track CS subscription namespace IDs (gitlab-org/gitlab!137220 - 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.