Support a callable model

[liorheber]

This update enhances the Agent class by allowing models to be specified as string, Model object, or callable.
This mirrors the flexibility already available for system prompts.

Key Features

• Flexible Model Configuration: Models can now be dynamically set using functions, offering more adaptability.
• Consistent Interface: Maintains backward compatibility with existing model provider interfaces.

Why This Matters
This change makes the Agent class more versatile, allowing dynamic model selection based on context, which is especially useful for complex workflows.

Assurance
All tests, linting, and type-checking pass.