Handle bigint helpers to consider columns passed being bigint

What does this MR do and why?

With Use bigint for new installations (#438124 - closed) new instances will have all IDs in bigint. This MR updates the helpers around bigint convertion to not re-create the conversion steps for such IDs.

Available bigint helpers:

1. initialize_conversion_of_integer_to_bigint -> does not generate _convert_to_bigint columns for existing bigint columns.
   • Updated docs to advise not to commit this schema changes to db/structure.sql
2. revert_initialize_conversion_of_integer_to_bigint -> not needed, as both remove_rename_triggers and remove_column safe deletes.
3. restore_conversion_of_integer_to_bigint -> after cleanup_conversion_of_integer_to_bigint the int4 columns would have been converted to int8, so it just carries on the usual process.
4. backfill_conversion_of_integer_to_bigint -> does not change the arguments and keeps using the full list of columns since it checks if _convert_to_bigint columns exist before doing the assignments.
5. revert_backfill_conversion_of_integer_to_bigint -> does not change the arguments and keeps using the full list of columns as (4).

Note:

There is no helper for swapping the column (N+1), I think it can be handled manually.

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Related to #498681 (closed)