Add generation_expression to information_schema.columns

In MySQL the information_schema.columns table has a generation_expression column containing the expression used for generated columns. In MariaDB this column is missing, which makes it more difficult for third party software to interplay with both MariaDB and MySQL by looking for this schema data in one location. Instead, it seems MariaDB only contains this information in the table's create statement which must be parsed — a non-trivial and fragile task.