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 84.21053% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.15%. Comparing base (1ba145b) to head (a1d021b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parser/mod.rs	85.33%	3 Missing and 8 partials ⚠️
src/ast/dcl.rs	83.92%	3 Missing and 6 partials ⚠️
src/ast/spans.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2417    +/-   ##
========================================
  Coverage   81.14%   81.15%            
========================================
  Files          42       42            
  Lines       33739    33866   +127     
  Branches    33739    33866   +127     
========================================
+ Hits        27379    27485   +106     
- Misses       2797     2804     +7     
- Partials     3563     3577    +14     

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