[big-query] Add support for struct types

BigQuery users often use the struct type for some use-cases.
We should validate whether Hasura can support the struct type and then come up design spec what a BigQuery struct type in Hasura would look like as a first step!

Reference:
https://cloud.google.com/bigquery/docs/reference/standard-sql/data-types#struct_type
https://cloud.google.com/bigquery/docs/nested-repeate

Read the linked BigQuery struct and nested/repeated data-type references first, then inspect the repository’s existing BigQuery integration; no file or entry point is named in the issue. Done means producing the requested feasibility assessment and a design specification for representing BigQuery structs in Hasura.

Written by the indexing model from the issue text.