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%.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[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!

[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

ℹ️ Googlers: Go here for more info.

[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