default_value_expression: str, Optional            Used to specify the default value of a field using a SQL expression. It can only be set for            top level fields (columns).            You can use a struct or array expression to specify default value for the entire struct or            array. The valid SQL expressions are:            - Literals for all data types, including STRUCT and ARRAY.            - The following functions:                `CURRENT_TIMESTAMP`                `CURRENT_TIME`                `CURRENT_DATE`                `CURRENT_DATETIME`                `GENERATE_UUID`                `RAND`                `SESSION_USER`                `ST_GEOPOINT`            - Struct or array composed with the above allowed functions, for example:                "[CURRENT_DATE(), DATE '2020-01-01'"]