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 81.36%. Comparing base (ada8314) to head (31663d7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2429      +/-   ##
==========================================
+ Coverage   81.28%   81.36%   +0.07%     
==========================================
  Files          42       42              
  Lines       33920    34016      +96     
  Branches    33920    34016      +96     
==========================================
+ Hits        27571    27676     +105     
+ Misses       2785     2773      -12     
- Partials     3564     3567       +3     

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[sabir-akhadov-localstack]

One non-blocking comment inline.

[sabir-akhadov-localstack]

PostgreSQL rejects IF NOT EXISTS for CREATE ROLE too, so this guard only makes the two spellings diverge: CREATE GROUP IF NOT EXISTS g now takes IF as the group name and fails with Expected: end of statement, found: NOT. Either drop the guard to keep parity with ROLE, or reject it with an explicit error.

[LucaCappelletti94]

Now it rejects consistently in both cases