Use migration new column as identity for backfill

[eminano]

Currently a migration with a backfill is not allowed on tables that do not have a unique not null column. Ideally if the migration requiring the backfill adds a unique not null column, the backfill should be possible.

Related conversation and test covering this case:
https://xata-hq.slack.com/archives/C05TR0HPQ9Y/p1709115340867789
#298

Investigate if it's possible to use a new column as the identity for a backfill (it would be full of NULL values at the time of the backfill).