add `index=True` option to `to_gbq`

[tswast]

Is your feature request related to a problem? Please describe.

I'd like to be able to control when the DataFrame Index/MultiIndex is included in the serialized BigQuery table.

Describe the solution you'd like

I'd like a boolean parameter index=True to enable writing the index column(s) to the BigQuery table. For backwards compatibility, we might need to keep this parameter False by default.

Describe alternatives you've considered

Folks can always call DataFrame.reset_index(drop=False) before uploading if they want this behavior, but I do think it'd be nice to be consistent with other I/O methods.

Additional context

• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.to_parquet.html (Has three options, where None is like True but serializes RangeIndex to metadata instead of as a column)
• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.to_csv.html (index=True by default. Also includes an index_label="string-here" parameter to override the index name)