Add Snowflake Support

[rechemendia]

Implement native support for Snowflake, including schema discovery and SQL generation, using the official Snowflake Python Connector.

Why:
Snowflake is a cloud-native analytical platform used by many enterprise teams. Adding support allows nlp2sql to be used in modern data stacks.

Tasks:

• Add SNOWFLAKE to DatabaseType enum
• Create snowflake_adapter.py to handle:
• Auth and connection via snowflake-connector-python
• Schema extraction using SHOW TABLES, DESCRIBE TABLE, etc.
• Adapt SQL generation to Snowflake-specific functions/syntax if needed
• Extend CLI support for --database-type snowflake
• Update README and usage examples