feat: add df.bigquery pandas accessor

[tswast]

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Fixes #<issue_number_goes_here> 🦕

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[tswast]

Not ready yet. Initial version works, but needs tweaking (unit tests fail, and dependence on column order is not great).

[tswast]

I believe I have addressed the unit test failures.

[tswast]

docs failure is because the pandas docs are currently down.

WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://pandas.pydata.org/pandas-docs/stable/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 522 Server Error: <none> for url: https://pandas.pydata.org/pandas-docs/stable/objects.inv

e2e failure appears to be a networking problem for remote functions:

FAILED tests/system/large/functions/test_remote_function.py::test_remote_function_via_session_vpc
FAILED tests/system/large/functions/test_remote_function.py::test_remote_function_via_session_custom_sa
FAILED tests/system/large/functions/test_remote_function.py::test_remote_function_via_session_custom_build_sa[fully-qualified-sa]
FAILED tests/system/large/functions/test_remote_function.py::test_remote_function_via_session_custom_build_sa[just-sa-email]
= 4 failed, 490 passed, 14 skipped, 6 xfailed, 6 xpassed, 476 warnings, 5 retried in 2030.24s (0:33:50) =

[tswast]

Thanks, @shuoweil ! FYI: I filed sphinx-doc/sphinx#14341 to investigate why the suppress_warnings configuration wasn't working as documented.