Add Column_definition::type_handler()

We'll change Column_definition as follows:

• remove the enum_field_types sql_type member, responsible for data type
• derive Column_definition from Type_handler_hybrid_field_type instead

This is needed to add new data types easier, including pluggable data types.
Eventually all data types inside the server will be identified by their Type_handler pointers, rather than a enum_field_types type code. This task is a step towards.