PostgreSQL: Support the VARIADIC function call argument marker

[LucaCappelletti94]

PostgreSqlDialect could not parse the VARIADIC argument marker in a function call, in any position:

Parser::parse_sql(&PostgreSqlDialect {}, "SELECT concat(VARIADIC a)");
// Err: sql parser error: Expected: ), found: a at Line: 1, Column: 24

The marker is added as FunctionArg::Variadic. Parsing is gated on Dialect::supports_variadic_function_argument, enabled for PostgreSQL and the generic dialect.

As in PostgreSQL, the marker is rejected on a wildcard operand, in combination with DISTINCT or ALL, and on any but the final argument.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.16%. Comparing base (bfc7d38) to head (efb39cc).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parser/mod.rs	86.84%	2 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2484      +/-   ##
==========================================
+ Coverage   81.09%   81.16%   +0.06%     
==========================================
  Files          42       42              
  Lines       33682    33813     +131     
  Branches    33682    33813     +131     
==========================================
+ Hits        27316    27444     +128     
- Misses       2796     2802       +6     
+ Partials     3570     3567       -3     

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