Support PostgreSQL ALTER DEFAULT PRIVILEGES

[LucaCappelletti94]

ALTER DEFAULT PRIVILEGES was not a recognised ALTER target, so it failed at the DEFAULT keyword.

The statement now parses into AlterDefaultPrivileges, carrying the optional FOR ROLE and IN SCHEMA lists and the abbreviated GRANT or REVOKE, so it round trips rather than being consumed and discarded.

I tried to maximize code reuse from analogous structs.
All of these PRs come from trying to parse pg_dump -s outputs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.57718% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.31%. Comparing base (ada8314) to head (1717eaa).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parser/mod.rs	87.77%	3 Missing and 8 partials ⚠️
src/ast/dcl.rs	85.96%	2 Missing and 6 partials ⚠️
src/ast/spans.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2417      +/-   ##
==========================================
+ Coverage   81.28%   81.31%   +0.03%     
==========================================
  Files          42       42              
  Lines       33920    34093     +173     
  Branches    33920    34093     +173     
==========================================
+ Hits        27571    27724     +153     
- Misses       2785     2791       +6     
- Partials     3564     3578      +14     

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[sabir-akhadov-localstack]

Two non-blocking comments inline.

[sabir-akhadov-localstack]

The current docs also list ON LARGE OBJECTS (PG 17+), so ALTER DEFAULT PRIVILEGES GRANT SELECT, UPDATE ON LARGE OBJECTS TO app fails here; a LargeObjects variant matched via parse_keywords(&[LARGE, OBJECTS]) would cover it.

[LucaCappelletti94]

Done

[sabir-akhadov-localstack]

DefACLOptionList in gram.y is an unordered list, so ALTER DEFAULT PRIVILEGES IN SCHEMA s FOR ROLE r GRANT SELECT ON TABLES TO app is valid PG but fails here with Expected: GRANT or REVOKE; looping over the two options until neither matches would accept it. pg_dump always emits FOR ROLE first, so non-blocking.

[LucaCappelletti94]

FOR ROLE and IN SCHEMA are now one ordered option list, so either order parses and prints as written, and repeatitions are rejected, as PostgreSQL does.