Interface to implement when creating a custom change that generates SQL. When updating a database,
implementing this interface is preferred over CustomTaskChange because the SQL can either be executed
directly or saved to a text file for later use depending on the migration mode used. To allow
the change to be rolled back, also implement the CustomSqlRollback interface. If your change requires sql-based
logic and non-sql-based logic, it is best to create a change set that contains a mix of CustomSqlChange and CustomTaskChange calls.