Summary

Add the core Async C Driver handle types: client, database, and collection, and perform the MongoDB handshake on connect.

Motivation

This is the foundational API surface for the Async C Driver. The remaining operation tickets under CDRIVER-4780 depend on these handles existing.

Acceptance Criteria

• Client, database, and collection handles can be created and destroyed.
• Handshake metadata identifies driver as Async C Driver.