SQL cell implementation with DuckDB

[rgbkrk]

Implement SQL cell execution with DuckDB integration and database connectivity.

Current State: SQL cells exist in schema but no execution capability
Target: Full SQL execution with DuckDB integration and database connections

Implementation Tasks:

• DuckDB integration - SQL execution via Runtime bridge
• Database connections - Connect to external databases (PostgreSQL, MySQL, etc.)
• Result visualization - Rich display of query results
• Python interop - Share data between SQL and Python cells
• Schema introspection - Browse database schemas and tables

Features:

• DuckDB-powered SQL execution
• External database connectivity
• Rich result display with tables and charts
• Data sharing between SQL and Python cells
• Database schema exploration
• Query result caching

TBD: DuckDB integration approach, database connection management, Python interop design