Track CS subscription namespace IDs

What does this MR do and why?

This MR adds X-Gitlab-Saas-Namespace-Ids header to the Code Suggestion requests sent to AI Gateway. This 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+.

AI Gateway counter part: Track gitlab saas namespace ids in snowplow event (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!465 - merged)

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