Implement BigQuery DATE_ADD/DATE_DIFF

Add support for BigQuery DATE_ADD/DATE_DIFF functions.

DATE_ADD(date_expression, interval) adds the interval to the date expression.
Example: DATE_ADD(DATE '2008-12-25', INTERVAL 3 DAY) would return '2008-12-28'.
BigQuery docs

DATE_DIFF(date_expression, date_expression2, time_unit) returns the whole number of time_unit between the first and second date expressions, with the result being negative if the first date is earlier than the second.
Example: DATE_DIFF(DATE '2008-12-25', DATE '2008-12-28', DAY) would return -3.
BigQuery docs