To keep a persistent backend storage for configuration, we will have to
keep a single source of truth. This involves supporting bi-directional
conversion between Config and Source.
This PR make the following changes:
* Embed Config in Source
* Add `ToConfig()` to extract Config from Source.
Update bigquery test to include column order for SELECT statement.
Update mindsdb tests to drop table before creating. The whole
integration test pause when there's a failure from any one of
integration tests. If the test pause after `CREATE` and before `DROP`,
the int test will fail when running it again.
