Feature: JSON and CSV output formats

[chriteixeira]

JSON Serialization Fix:

• Fixed execute_sql function to return valid JSON format instead of Python repr format (e.g., Decimal('75000.00') → 75000.0)
• Added custom JSON serializer that properly handles PostgreSQL data types:
  • Decimal → float
  • datetime/date → ISO string format
  • timedelta → total seconds
  • bytes/binary → base64 encoding
  • UUID → string representation

CSV Output Format:

• Added new output_format parameter to execute_sql function with options: "json" (default) or "csv"
• Implemented format_csv_response() function that converts query results to proper CSV format
• CSV format preserves decimal precision and handles complex data types appropriately