Pass user preference about caching to the Workflow Service

What does this MR do and why?

This MR passes ai_gateway_allow_conversation_caching feature flag and x-gitlab-model-prompt-cache-enabled header to DWS, which is required for gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!3722 (merged).

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/577652+

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