WL#8433: Separate DD commands from regular SQL queries in the parser grammar

This is a parser refactoring WL.

The current main parser grammar is a mix of regular SQL statements and special Data Dictionary-related declarations, for example:

statement:

       ...
       | parse_gcol_expr
       | partition_entry
       ...

where parse_gcol_expr and partition_entry aren't valid SQL statements but data definitions from DD. Thus, when we parse each "statement", we have to check its origins in some tricky way.

Suggestion: it would be nice to separate such grammar rules from regular SQL with the help of lexical scanner.

See requirements at the umbrella WL#8067 page.

See specifications at the umbrella WL#8067 page.

See specifications at the umbrella WL#8067 page.