Extension support and starter sample

[rodrigobr-msft]

This pull request introduces a new "extension-starter" sample for the SDK, demonstrating how to build custom extensions and event handlers on top of the SDK. It also refactors the core application routing logic to use a new RouteList structure, simplifies type hints, and improves the extensibility of the routing system. The most important changes are grouped below:

New Extension Starter Sample and Documentation:

• Added a new extension-starter sample in test_samples, including example extension agent code, custom event models, and a sample application setup to illustrate how to extend the SDK with custom routes and event handling. [1] [2] [3] [4] [5] [6]

Core Routing System Refactor:

• Refactored the routing system in AgentApplication to use a new RouteList class instead of a plain list, improving manageability and extensibility of routes. The add_route method is now the central way to register new routes, and all decorator-based registration methods (activity, message, etc.) delegate to it. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Type Hint and Import Improvements:

• Standardized type hints for lists (using list[...] instead of List[...]) and updated imports for routing-related types across the core hosting modules for better clarity and type safety. [1] [2] [3] [4] [5] [6] [7]

Minor Cleanups:

• Removed unused imports and cleaned up redundant code in the core modules.

These changes collectively make it easier to build, register, and manage custom event handlers and routes in SDK-based agent applications, while providing a clear sample for extension development.

[cleemullins]

Reviewed in person