PostgreSQL ALTER FUNCTION / ALTER AGGREGATE

[LucaCappelletti94]

What this PR introduces

• Adds parser + AST support for ALTER FUNCTION and ALTER AGGREGATE statements.
• Adds support for operations:
  • RENAME TO
  • OWNER TO
  • SET SCHEMA
• Adds ALTER FUNCTION action parsing (IMMUTABLE/STABLE/VOLATILE, null-input modes, LEAKPROOF, security, PARALLEL, COST, ROWS, SUPPORT, SET, RESET, optional RESTRICT).
• Adds function-level [NO] DEPENDS ON EXTENSION handling.
• Adds aggregate signature support for:
  • (*)
  • (argtypes)
  • (argtypes ORDER BY argtypes)
  • (ORDER BY argtypes)

PostgreSQL parity adjustments

• Allows unambiguous argless ALTER FUNCTION name action forms.
• Restricts [NO] DEPENDS ON EXTENSION to ALTER FUNCTION (not aggregate).
• Tightens aggregate argument rules by rejecting unsupported modes/defaults (OUT, INOUT, VARIADIC, DEFAULT, = in aggregate signatures).
• Adds VARIADIC as a function argument mode in parser/AST.

Tests

• Adds a focused PostgreSQL test matrix for ALTER FUNCTION / ALTER AGGREGATE variants.
• Includes both positive and negative parity cases and canonical output assertions.
• Existing related PostgreSQL alter/create-function tests remain passing.