[ADAP-1010] Support for new agate data type in BigQuery

[gshank]

dbt Core changes

To support integers in the "show" command, a new agate data_type, Integer, was added in dbt Core in core/dbt/clients/agate_helper.py. The agate table creation code is also used when creating empty tables for seeds, such as when a seed file has headers but doesn't have any rows. Because the "convert_agate_type" method did not include the new Integer type, "None" was returned and the sql generated by the macro "create_csv_table" was incorrect.

dbt Core pull request dbt-labs/dbt-core#9004 adds a new "convert_integer_type" method and adds Integer to the list of agate types in "convert_agate_type".

Adapter changes

This pull request will pull in the new empty seed test and provide a "convert_integer_type" method.