Allow querying remote MySQL data sources

The ability to federate queries to different data sources would be useful to customers for scenarios such as when remote data needs to be combined with local data for query results.

One approach for this is providing a new table function that can query a remote MySQL data source. This provides an explicit interface for querying a remote MySQL server, instead of a more complex, implicit approach where the query planner must identify which tables are remote in a query.

BigQuery provides this functionality through its [`EXTERNAL_QUERY()` function](https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/federated_query_functions#external_query).

```sql
SELECT * FROM myLocalTable
LEFT JOIN EXTERNAL_QUERY('us.prod_remote_db', 'SELECT * FROM myRemoteTable') T ON myLocalTable.id = T.id
WHERE T.id = 1;
```

Implementation of the table function should be fairly straightforward. One design issue we would need to solve is authentication params for the remote server. BigQuery addresses this by allowing admin users to create a connection resource that contains all the connection information, then grants users privileges to use that connection resource in external queries.