Add first class for new configuration system

[SamRemis]

This PR introduces a configuration system for AWS clients that solves the challenge of managing configuration from multiple sources while maintaining proper precedence, validation, and auditability. AWS SDKs need to handle configuration from constructor parameters, environment variables, config files, and credentials files with a well-defined precedence order, while also tracking where each value originated for debugging and complex resolution logic.

The implementation provides async configuration resolution to handle file I/O operations, comprehensive type validation that works around Python's protocol type limitations, and extensible custom validation for complex types like endpoint URIs. Each configuration value is wrapped with source metadata, enabling users to understand where a particular value came from.

The system is designed to be easily extensible for new configuration fields while maintaining type safety and IDE support. It handles the complexity of AWS configuration requirements while providing a clean interface for client initialization and runtime configuration access.