Add X-Gitlab-Host-Name to the AI Gateway header

What does this MR do and why?

This MR adds X-Gitlab-Host-Name to the request header to the AI Gateway. This is requested by https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/277+.

• AI Gateway counter part: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!462 (merged)
• Snowplow schema counter part: https://gitlab.com/gitlab-org/iglu/-/merge_requests/98

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