feat: add column and table extraction for complex queries with subqueries

[label-hook]

Summary

Adds functionality to extract column names along with their source table names from complex SQL queries including nested subqueries. This addresses the need to analyze column-to-table relationships in sophisticated SQL statements with JOINs, aliases, and multiple nesting levels.

Changes

• Added extract_columns_and_tables() function in sqlparse/utils.py that recursively traverses the SQL AST to identify column references and their source tables
• Added helper functions for identifying column references, table aliases, and building comprehensive column-to-table mappings
• Created example script examples/extract_columns_and_tables.py demonstrating usage with complex queries including the provided test case
• Updated existing example examples/extract_table_names.py to reference the new column extraction functionality
• Added comprehensive test suite tests/test_column_extraction.py covering various SQL patterns including subqueries, JOINs, aliases, and edge cases

Testing

• All existing tests continue to pass
• New test suite covers:
  • Simple SELECT statements with column extraction
  • Complex queries with nested subqueries (including the provided example)
  • JOIN operations with table aliases
  • Column aliases and qualified column names
  • Edge cases with CTEs and various SQL dialects
• Verified correct extraction of columns ('Gender', 'Ethnicity', 'count') and their source tables from the complex example query

Closes #25

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Closes #25