feat: Add SQL support for multi-condition joins

[makarandhinge]

Todo #545

• Enhanced WayangMultiConditionJoinVisitor with SQL implementation
• Updated JdbcJoinOperator to generate AND clauses for multi-column joins
• Added test coverage for multi-condition join SQL generation
• Maintains backward compatibility with single-condition joins

[mspruc]

Hi, it doesn't seem like your test case tests the implementation

[makarandhinge]

Thank you @mspruc for the feedback! You’re right
the current test only checks the SQL string generation and doesn’t actually verify execution. Right now it covers things like SQL generation with AND clauses and correct table and column names. We could improve it by running the generated SQL and checking the results, such as making sure the correct number of rows is returned, joins match properly (for example, order_id=1 and customer_id=100), and data from both tables is combined correctly. Would you like me to add a full integration test to validate execution and results?