Add desired_sharding_keys to sync event tables

What does this MR do and why?

This MR adds the desired sharding_key for the p_ci_finished_build_ch_sync_events and p_ci_finished_pipeline_ch_sync_events tables.

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

Part of #459997 (closed) and #470152 (closed)

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