Add migration helpers to revert conversion to bigint

What does this MR do?

Add two new migration helpers:

• revert_initialize_conversion_of_integer_to_bigint
• revert_backfill_conversion_of_integer_to_bigint

to help with PK migration.

There is also nother helper method added (convert_to_bigint_column) that's used internally.

See #328600 (closed).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Related to #328600 (closed)