Transformers - Add FIRST/LAST functions that ignore nulls

[marcus-snx]

• add FIRST/LAST agg functions in postgres db (https://wiki.postgresql.org/wiki/First/last_(aggregate))

In Postgres there is no IGNORE NULLS for window functions (unlike e.g. Oracle or BigQuery), but they can be recreated using aggregates. Using these simplifies the queries a bit without impacting speed.

The code below takes the last non-null value in the window. The coalesce is there in case there are nulls at the beginning of the ordering.

coalesce(
    last(debt) over (
        partition by dim.collateral_type, dim.pool_id 
        order by dim.ts
        rows between unbounded preceding and current row
    ),
    0
) as debt