Is your feature request related to a problem? Please describe.
It appears that the BigQuery DBAPI does not support multi-row INSERT batching for more performant python-based DML transactions. Current executemany INSERT statements are executed one at a time, leading to massive slowdowns in batch INSERT DML operations.

Example multi-row insert from BiqQuery documentation:
https://cloud.google.com/bigquery/docs/reference/standard-sql/dml-syntax#insert_examples

INSERT dataset.Inventory (product, quantity)VALUES('top load washer', 10),      ('front load washer', 20),      ('dryer', 30),      ('refrigerator', 10),      ('microwave', 20),      ('dishwasher', 30),      ('oven', 5)

 
Describe the solution you'd like
Add multi-row INSERT batching support.
MySQL DBAPI example:https://github.com/PyMySQL/PyMySQL/blob/main/pymysql/cursors.py#L194

Describe alternatives you've considered
I will probably crudely make a patch to my sqlalchemy-bigquery DBAPI cursor to enable this support for my project that needs this performance boost for my ORM based application.

Additional context
sqlalchemy-bigquery related ticket:googleapis/python-bigquery-sqlalchemy#497
sqlalchemy related discussion:sqlalchemy/sqlalchemy#12038