Add a py.typed file for submodule

[crwilcox]

To be in accordance with PEP 561 we should add a py.typed file to google/cloud/bigquery/py.typed

The file can be empty but does need to be present for some type checkers.

[tswast]

Please don't until #987 is fixed.

[crwilcox]

Do note, you may need to run mypy, pyright, pytype against our samples before merging. It will ensure that we aren't exposing users to failing type checks.

Ideally we would add this to CI as well, to avoid future breakage.

[plamut]

Update - we will adjust the annotations to make mypy happy, but we will only add py.typed to the v3 branch (and ship it in 3.0.0).

Even if type checks break for some people, it will only happen in the next major release, which should not happen automatically, if the BigQuery dependency is pinned to < 3.x (as it should be).

(note to self - besides adding the py.typed file itself, we must also list it setup.py, option package_data, so that it actually gets installed)

[crwilcox]

https://github.com/googleapis/python-bigquery/blob/main/setup.cfg has a pytype section. Is that tied in to packaging? I am unsure.

[plamut]

Mypy is happy now, unblocking.

We will only add py.typed to the v3 branch, though, as discussed offline.

[plamut]

Closed in #1058 (merged into the v3 branch).