Add column_name to batched_background_migrations Admin API

What does this MR do and why?

Add column_name to Admin batched_background_migration API's response, so that chatops can display this field column_name in slack response.

Currently, chatops displays fields table_name and job_class_name. However, this is not enough for a dev to tell which one is which when there are more than one batched background migrations against different columns with the same table and job class name.

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