snapshot invalidate_hard_deletes feature

[dstsimokha]

Is your feature request related to a problem? Please describe. invalidate_hard_deletes=True for snapshots not working with CH because of its default non-nullable column

Describe the solution you'd like dbt general default__snapshot_staging_table macro can be modified in two places by changing {{ strategy.unique_key }} to toNullable({{ strategy.unique_key }}) - in snapshotted_data and deletes_source_data CTEs