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<STRUCT<price FLOAT64, balance FLOAT64, currency_iso STRING, ...>> 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