Parse BigQuery federated queries

Hi all,

In BigQuery, it is possible to execute queries against Cloud SQL instances (GCP managed database), using "federated queries" see example [here](https://cloud.google.com/bigquery/docs/cloud-sql-federated-queries#example), using the `EXTERNAL_QUERY` function, example:

```
SELECT c.customer_id, c.name, rq.first_order_date
FROM mydataset.customers AS c
LEFT OUTER JOIN EXTERNAL_QUERY(
'us.connection_id',
'''SELECT customer_id, MIN(order_date) AS first_order_date
FROM orders
GROUP BY customer_id''') AS rq ON rq.customer_id = c.customer_id
GROUP BY c.customer_id, c.name, rq.first_order_date;
```

Has anyone else had success with parsing these BigQuery federated queries? I am interested in extracting what tables are referred, both in the BigQuery SQL and the referred tables from the federated query. One option could be to add the function `EXTERNAL_QUERY` to my Analyzer and analyze the federated query first and use it somehow in the context of the entire query, but maybe someone has a better solution as to how I can parse these queries together? Thanks

Start by reviewing the Analyzer and the EXTERNAL_QUERY function described in the issue, alongside the linked BigQuery federated-query example. Define how table references from both the outer BigQuery SQL and embedded Cloud SQL query should be represented; done means extracting both sets from a federated query.

Written by the indexing model from the issue text.