Added support for structs and modes (repeated and required) for BigQuery

[faahren]

Description & motivation

The BigQuery schema declaration was not supporting nested fields (STRUCT, or RECORD) as well as the REPEATED option that gives possibilty to store an array. Those are interesting when linking external tables more complex than CSV (Json, Parquet, etc)

Also, all fields were nullable by default and we could not set them to REQUIRED.

I updated the macro to allow full support for those 3 features
I have not updated the readme but added a sample for bigquery in the dedicated file.
I can't add a test as I do not have access to the bucket containing external samples (To add a nested JSON for example)

Checklist

• [ X] I have verified that these changes work locally
• [X ] I have updated the README.md (if applicable ) SAMPLES ADDED TO THE SAMPLE FILE
• I have added an integration test for my fix/feature (if applicable)

[faahren]

@jeremyyeo Any idea when someone can look into this ?

Thanks