Add runner_type, sharding_key_id columns to ci_runner_machines

🚨 NOTE: There is urgency in getting this MR merged soon, since the batched background migrations 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 a future MR, we'll be creating a partitioned copy of the table with an automated trigger to copy data from ci_runner_machines 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 the runner_type and sharding_key_id columns to the ci_runner_machines table.
• Adds logic to populate runner_type and sharding_key_id when a runner manager is created or updated.
• Adds a post-deployment migration to backfill the fields from ci_runners.

Changelog: added

Part of #497526 (closed)

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