BigQuery: Set BQ storage client user-agent when in Jupyter cell

[plamut]

Closes #8696

This is a follow-up to #8713, it adds the same user-agent string to the optional BQ storage client, if the latter is used.

How to test

See issue description and the subsequent comment about adding the user-agent info to the other client, too.

[plamut]

When running unit tests locally, I noticed that one of them fails here. The reason seems to be that float("nan") is converted to None by pyarrow, instead of the expected float("nan"). It also happens on the latest master, thus the failure seems unrelated to this PR.

My Pandas and pyarrow versions:

pandas==0.24.2
pyarrow==0.13.0

Update: The same happened in Kokoro checks, did not observe this yesterday...

[tswast]

Should we make this __import__ mock into a test utility (at https://github.com/googleapis/google-cloud-python/tree/master/test_utils/test_utils or maybe https://github.com/googleapis/google-cloud-python/blob/master/bigquery/tests/unit/helpers.py)?

Seems generally useful, and also something I'd like to see tests for.

[plamut]

I'd say we put it in bigquery test utils for now, and ask other stakeholders if they are fine with promoting it to core test utils.
(there might be additional features/generalizations requested, but that would not block this particular PR)

[plamut]

BTW, whom can I ask if the new helper is fit for promotion, who owns the core?

(if there isn't one and we can do it at our own discretion, please let me know)

[tseaver]

BTW, whom can I ask if the new helper is fit for promotion, who owns the core?

(if there isn't one and we can do it at our own discretion, please let me know)

I would say add a separate PR promoting it, and tag @busunkim96 for review.