Add sharding column to merge_request_diff_commits partitioned table (Step 1.5)

Per &12795 (comment 1925659540)

1. Add the eventual sharding key column to the new partitioned table now. We will leave this as a nullable field for now even though we would eventually want this to be required/non-nullable.
2. Make sure that the partitioned tables are marked with desired_sharding_key in their respective YML