Split up the int column conversion helpers

What does this MR do?

Related: #300915 (closed), #288004 (closed)

Split the migration helper used to kickoff an integer-to-bigint column conversion into two separate methods that will be called as two independent steps.

The existing method (and first step) initialize_conversion_of_integer_to_bigint will add the new temporary bigint column, and install the triggers to keep the column in sync with the existing column.

The new method (and second step) backfill_conversion_of_integer_to_bigint will create the batched_background_migration record that will be used to run the backfill of the temporary column.

Keeping this as two steps allows the schema changes to be made in a pre-deployment migration, while the background migration can be added in a post-deployment migration.

Also update the helpers to run the background migration inline in test/dev environments.

The helpers will first be used in the follow up MR: !51332 (merged)

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