PostgreSQL: parse CREATE GROUP and DROP GROUP

[LucaCappelletti94]

PostgreSQL accepts GROUP wherever it accepts ROLE in these two statements, but the parser rejected it, so a schema dump containing CREATE GROUP could not be read at all.

The keyword is preserved rather than rewritten to ROLE. It looks like a pure alias in PostgreSQL, but Amazon Redshift has user groups as objects distinct from roles, so folding the two would change what the statement means there.

I will also add ALTER GROUP in a following PR, so as to keep this one smaller.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.19%. Comparing base (1ba145b) to head (f391d2c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2429      +/-   ##
==========================================
+ Coverage   81.14%   81.19%   +0.04%     
==========================================
  Files          42       42              
  Lines       33739    33764      +25     
  Branches    33739    33764      +25     
==========================================
+ Hits        27379    27413      +34     
+ Misses       2797     2785      -12     
- Partials     3563     3566       +3     

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