Currently when adding columns with the declarative schema changer, we fallback to the legacy schema changer when sequence expressions are observed inside newly added columns this was done because:

1. We didn't support properly surfacing errors from the backfiller (this was addressed in the original PR)
2. We lacked support for updating error related telemetry during executing

Once these two points are addressed we can enable support for the declarative schema changer for all expressions.

Jira issue: CRDB-16067