feat: add "columns" as an alias for "col_order"

[tswast]

One thing I'd like is to align read_gbq with read_sql and read_sql_table. Those methods call what we call "col_order", "columns". See: https://pandas.pydata.org/docs/reference/api/pandas.read_sql.html

[tswast]

Note: When reading via the BQ Storage API, we could potentially use the columns argument here: https://cloud.google.com/bigquery/docs/reference/storage/rpc/google.cloud.bigquery.storage.v1#google.cloud.bigquery.storage.v1.ReadSession.TableReadOptions.FIELDS.repeated.string.google.cloud.bigquery.storage.v1.ReadSession.TableReadOptions.selected_fields