Add data migration to migrate ai usage tables in CH to ai_usage_events CH table

Summary

As part of our effort to bring generalized AI usage data to production-ready state in scope of this issue we need to

Proposal

Add data migration to migrate ai usage tables in CH to ai_usage_events CH table. See PoC for guidance.

Technical requirements

  1. Cover all Ai events we have in CH. (at the moment this issue was created it's CS, Duo Chat, Troubleshoot Job)
  2. Split migration queries per week to avoid timeouts.
  3. Do not merge before we enable double-write for existing events.
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