feat: add default value expression to SchemaField

[aribray]

Adds default_value_expression to SchemaField.

        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'"]

Googlers see 241007941

[tswast]

FYI: @shollyman , this was a breaking change. Python allows for this to be both positional and keyword argument unless you use *, syntax added to Python 3.0 https://peps.python.org/pep-3102/