Add ai_context_compaction feature flag

What does this MR do and why?

Add ai_context_compaction feature flag to gradually rollout the context compaction feature tracked in this issue gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1863, which is currently in development. And we want to rollout to a small set of users first to collect feedback.

Rollout issue #590815

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Before	After

How to set up and validate locally

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