Add DocumentDB DuckDB extension

[sandeepsnairms]

Summary

This PR adds the DocumentDB DuckDB extension implementation, build wiring, and the initial validation evidence for publication review.

Included work

• DuckDB extension loader and CMake wiring
• DocumentDB connection, schema, and scan scaffolding
• SQL function registration for version and collection metadata
• smoke tests for the extension behavior
• usage and architecture documentation

Validation

Verified in the current repo state:

• native build succeeds
• Linux Docker build path succeeds via DuckDB extension-ci-tools
• runtime SQL functions work as expected in the DuckDB shell

Notes

This is a publication-ready review branch for the extension work and the associated validation trail.

[hossain-rayhan]

Looks Good as first scaffolding. Let's move forward addressing the minor comments.