Add MongoDB support

[quinnjr]

Description

Add MongoDB support to Prax ORM using the mongodb crate.

Tasks

• Create prax-mongodb crate
• Implement document mapping
• Add query translation layer
• Support aggregation pipelines
• Add connection pooling
• Write tests and documentation

Related

• Uses mongodb crate for async operations
• Follow patterns from existing drivers (postgres, mysql, sqlite)

Acceptance Criteria

• CRUD operations work
• Filtering translates to MongoDB queries
• Relations work with references
• 90%+ test coverage