Snowflake: Add support for ->> (pipe) operator for chaining SQL stmts

[BenSatori]

The ->> (flow/pipe) operator chains multiple SQL statements in Snowflake.

Before: Parser rejected pipe chains like SELECT * FROM t1 ->> SELECT * FROM $1

After: Parser correctly handles:

• Chained statements with ->> operator
• Pipe result references ($1, $2, $3, etc.)
• Round-trip SQL serialization

Example:

SELECT * FROM users WHERE active = true ->> SELECT COUNT(*) FROM $1

Changes:

• Added Statement::Pipe AST variant for chained statements
• Added TableFactor::PipeResultScan for $n result references
• Extended Snowflake dialect with pipe operator support
• Comprehensive test coverage (2 test functions, multiple edge cases)

Snowflake docs:
https://docs.snowflake.com/en/sql-reference/operators-flow#pipe