Destination tables not respecting nullable/required modes specified in schema.json

While interim tables reflect the column modes specified in a supplied schema.json, these modes do not make their way to the final table definition.

**To Reproduce**
Steps to reproduce the behavior:
1. deploy function to your project
2. copy last_sale config files from examples to `/bqds`
3. copy last_sale.csv to bucket as `marketdata.last_sale.csv`
4. despite `last_sale.schema.json` having columns specified as `REQUIRED`, the ultimate destination table (`marketdata.last_sale`) shows all columns as being `NULLABLE`

Desired behavior is for all columns being mapped directly to the destination table to inherit the mode spec from their schema definition.

Thanks for the report @michaelwsherman