Add lifetime parameter to BorrowedToken for zero-copy tokenization

This change introduces a lifetime parameter 'a to BorrowedToken enum
  to prepare for zero-copy tokenization support. This is a foundational
  step toward reducing memory allocations during SQL parsing.

  Changes:
  - Added lifetime parameter to BorrowedToken<'a> enum
  - Added _Phantom(Cow<'a, str>) variant to carry the lifetime
  - Implemented Visit and VisitMut traits for Cow<'a, str> to support
    the visitor pattern with the new lifetime parameter
  - Fixed lifetime issues in visitor tests by using tokenized_owned()
    instead of tokenize() where owned tokens are required
  - Type alias Token = BorrowedToken<'static> maintains backward
    compatibility

tests/sqlparser_postgres.rs

@@ -2045,7 +2045,7 @@ fn parse_pg_returning() {
 fn test_operator(operator: &str, dialect: &TestedDialects, expected: BinaryOperator) {
     let operator_tokens =
         sqlparser::tokenizer::Tokenizer::new(&PostgreSqlDialect {}, &format!("a{operator}b"))
-            .tokenize()
+            .tokenized_owned()
             .unwrap();
     assert_eq!(
         operator_tokens.len(),