More bugs with Avro format support in BigQueryType writes

[clairemcginty]

The new GenericRecord default format still has a few bugs:

• Doesn't support doubly-nested optional record fields
• Doesn't support Elitzur validation types -- the schema converter doesn't take the OverrideTypeProvider into account and tries to encode the record type instead of the underlying primitive
• Timestamp representation works for Storage write API but not FileLoads API

tbh, there are enough bugs found so far that I think we should try to restore TableRow format at the default and make GenericRecord opt-in; the schema converter needs more flexibility. Maybe we can eventually switch to Magnolify for schema conversion or write our own.