…rom uri

* Specify BigQuery API endpoint using query param again.

* Add instructions for adding a Google Cloud Project Service Account
  for testing against actual Google BigQuery.

* Add BigQuery-compatible migrations as test fixtures.

* Change `con` to `conn` to be consistent with the `sql.Conn` type name
  and `database/sql` example code naming convention, and spell out
  `driverConn` where it's used to clearly distinguish between the
  `sql.Conn` and the underlying driver connection type.

* Introduce function to return driver connection's entire
  `bigQueryConfig` instance, rather than having separate functions for
  each field within that type.

* Simplify `DumpSchema()` using the BigQuery-provided DDL instead of
  generating it ourselves.

* Revert implementation that required dbmate's bigquery-schemed URLs to
  pass the project ID in the path portion of the URL instead of the
  hostname portion of the URL, and instead just pass through the URL to
  the underlying driver as-is.

* Add tests that can selectively run against the actual Google BigQuery
  service, for functionality that is not yet implemented in the emulator.