Add desired_sharding_key to p_ci_finished_build_ch_sync_events table

What does this MR do and why?

This MR adds the desired_sharding_key for the p_ci_finished_build_ch_sync_events table. It also adds a LFK to the p_ci_builds table.

https://docs.gitlab.com/ee/development/cells/index.html#define-a-desired_sharding_key-to-automatically-backfill-a-sharding_key

Part of #459997 (closed)

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