feat: Add py.typed for PEP 561 compliance

[jdufresne]

Type annotations were added in commit
f8d4aaa. For these annotations to be
useful by library users, the package should install a py.typed file.
This tells mypy and other tools to consume and use these types.

For more details, see:
https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages

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[jdufresne]

Is the CLA required for something so trivial?

[plamut]

@jdufresne Thank you for your contribution!

CLA is required, yes, regardless of the contribution size, otherwise the bot will not allow to merge a PR.

[jdufresne]

@googlebot I signed it!

[plamut]

FWIW, that failed system test is known flakiness.

@tswast We now actually have the job ID in the logs to investigate, I have shared it with you privately.

[plamut]

I would just suggest putting the following content in the py.typed file for consistency, similarly to bigquery_v2/py.typed and py.typed files in other Python client libraries:

# Marker file for PEP 561.
# The google-cloud-bigquery package uses inline types.

[jdufresne]

@plamut Thanks for the review. I've applied the suggestion to the latest revision.

[plamut]

Looks good as far as I'm concerned. And thanks again!