New dependency on `pyarrow` introduces heavyweight `numpy` sub-dependency

[di]

Is your feature request related to a problem? Please describe.
The new dependency on pyarrow, introduced in #1178, creates a new sub-dependency on numpy. Without fully understanding why these dependencies were introduced, a required dependency on numpy feels unnecessarily large for this library.

Describe the solution you'd like
Make the pyarrow and numpy dependencies optional (via extras).

Describe alternatives you've considered
Pin my usage of google-cloud-bigquery back to a version that does have these dependencies, or find a way to remove it entirely.

Additional context
The dependency on numpy in pyarrow: https://github.com/apache/arrow/blob/4a90e3994fc9fc10b968ab3439dec636385dec22/python/setup.py#L589-L591

(PS, thanks for your work on this library!)

[tswast]

Possibly a duplicate of #1142

[tswast]

For some background, the reason we added pyarrow as a required dependency is that we kept getting issues from folks installing incompatible versions of pyarrow. Perhaps there's another way to address this, but at the moment, pip doesn't do anything to help with incompatible versions in "extras" unless those extras are explicitly installed.

[majorgilles]

Concur with the post, we were using this as a replacement over the now bloated google-api-python-client and will be forced to pin this to the latest non problematic version because we use this from an AWS lambda environment which have very specific restricions in terms of deployment size

[tswast]

If someone were to send a PR to do this, I'd be open to it, given the extra dependency does appear to block more use cases than I anticipated.

We'd want to:

• Restore optional checks for pyarrow and google-cloud-bigquery-storage, which were removed in deps!: BigQuery Storage and pyarrow are required dependencies #776
• Restore guards against bad pyarrow versions: fix: guard imports against unsupported pyarrow versions #934
• Add pyarrow and google-cloud-bigquery-storage checks to verify_pandas_imports

  python-bigquery/google/cloud/bigquery/_pandas_helpers.py

  Line 987 in 7897b55

  def verify_pandas_imports():

[tswast]

To implement this, we'd basically want to revert #776. I doubt a simple "revert" will be sufficient at this point as 2.x diverged a bit from 3.x.