feat: add BigQuery storage client support to DB API

[plamut]

Closes#16.

The title says it all. Details in the ticket description.

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[tswast]

General structure LGTM. Replacing self._query_data with an iterable that calls the BQ Storage API under the covers is exactly what I imagined.

Once the PR is ready, let's have @shollyman or someone he delegates to do the final review.

[plamut]

The coverage failure is due to Python 2.7 unit tests are not run on Kokoro:

Session unit-2.7 skipped: Python interpreter 2.7 not found.

The tests pass just fine locally, and the total coverage is 100%.

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[plamut]

@tseaver As a co-author of one of the changes, please just appease the CLA bot, thanks!

[shollyman]

Thanks for this. We'll want to update this from v1beta1->v1 in the near future, not sure how you'd like to track that.

[plamut]

We'll want to update this from v1beta1->v1 in the near future, not sure how you'd like to track that.

@shollyman Track in the sense of the tasks that need to be done for the promotion to GA? A GitHub issue for it would be just fine.

[plamut]

@tseaver Friendly ping, the CLA bot still needs confirmation from you. Thanks!

[tseaver]

@googlebot I signed it!

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[yan-hic]

Can usage for this be updated at https://github.com/mxmzdlv/pybigquery/blob/master/README.rst#usage ?
Not sure how to enable storage API when loading in superset.

[yan-hic]

To be closed as per googleapis/python-bigquery-sqlalchemy#61 (comment) ?

[plamut]

@yiga2 Sorry, I'm not following? What should be closed, this PR has been merged?

[yan-hic]

My bad - meant to close googleapis/python-bigquery-sqlalchemy#41