refactor: Backfill materialized columns by column reference instead of property

[tkaemming]

Problem

Some properties will have multiple columns associated with them during the transition period when rematerializing columns as nullable, so simply providing a property source column and property name will no longer be specific enough to identify a particular column that needs to be backfilled.

More context available in #26651.

Changes

• Make backfill_materialized_columns take Iterable[MaterializedColumn] instead of list[tuple[PropertyName, TableColumn]] and update all relevant call sites.
• Also updated materialize to return a MaterializedColumn instead of ColumnName | None to get rid of some annoying workarounds to appease the type checker.

Does this work well for both Cloud and self-hosted?

N/A

How did you test this code?

Updated tests.