Add sharding_key_id to ci_runners table

🚨 NOTE: There is urgency in getting !167389 (merged) and this MR merged soon, since they unblock batched background migrations that must make it to %17.5 (which is a required stop), so we can finalize them in %17.6 🚨

What does this MR do and why?

The changes in this MR are needed because in !166308, we'll be creating a partitioned copy of the table with an automated trigger to copy data from ci_runners to the partitioned table. If we don't have all the required columns, the check constraint will fail on the target table.

This MR:

• Adds a sharding_key_id column to the ci_runners table.
• Adds logic to populate sharding_key_id when a runner is created, registered, or reassigned to a different owner project.
• Adds a post-deployment migration to backfill the field from ci_runner_namespaces and ci_runner_projects.

Changelog: added

Part of #493256 (closed)

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