Send X-Gitlab-Realm header to AI gateway

What does this MR do and why?

In order to split SaaS from self-managed traffic when requesting code suggestions, we are currently tracking a gitlab-realm field in the AI service access token payload. We did this because in the first iteration, IDEs would go straight to this service without going through their respective Rails instances first.

Since this has changed and we now want IDEs to target Rails instead, it is simpler and easier to work with an HTTP header field sent by gitlab-rails to the AI service instead. The header field added is X-Gitlab-Realm. It can be used for tracking events like these: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!282 (merged)

We need to maintain the token field for now since we will allow the old access path for a while longer.

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