feat: Validate custom authentication path and method in auth_config

[Iamsdt]

This pull request improves the robustness of custom authentication module loading in the agentflow_cli by adding validation and handling for file paths specified in the authentication configuration. The main changes ensure that both the method and path are provided, and that the specified file path exists before attempting to load the module.

Validation and error handling for authentication configuration:

• Added checks to ensure both 'method' and 'path' are specified in auth_config, raising a ValueError if either is missing.
• Implemented logic to extract and normalize the file path from the auth_config['path'], handling both filesystem paths and dotted module paths, and verifying that the file exists before loading. Raises a ValueError if the file does not exist.
• Imported os and Path from pathlib to support the new file path handling logic.