WL#8435: True bottom-up server parser: cleanup and refactoring column definition stuff

This WL is a part of umbrella refactoring WL#6707 "Refactor MySQL server parser to build the AST in a natural bottom-up way".

The WL will cleanup and refactor:

• column type definitions in CREATE/ALTER TABLE statements,
• PARSE_GCOL_EXPR pseudo-SQL statement (data definition in .frm).

See requirements at the umbrella WL#8067 page.

See specifications at the umbrella WL#8067 page.

See specifications at the umbrella WL#8067 page.