Support generating the data model for nested records in BigQuery

**Describe the bug**
I have a bigquery schema that uses nested repeated records, any reference of a field in the nested record throws an error like this:
```
Error: Error: Cannot access field balance on a value with type ARRAY> at [5:41]
```

I think for this to be solved cube.js needs to use the `UNNEST` sql keyword to flatten the array of records:
https://stackoverflow.com/questions/39109817/cannot-access-field-in-big-query-with-type-arraystructhitnumber-int64-time-in

The issue was mentioned at the end here, but it was closed so I opened a new one: #182