Add support for subscription events to WebView's tRPC

[tnaum-ms]

This PR introduces enhancements to the appRouter and the WebviewController to enable strongly typed event subscription support within a webview.

fixes #2516

Enhancements and new features:

• Subscription Procedures:

  • Introduced demoBasicSubscription and demoComplexSubscription to demonstrate streaming data from the server to the client using asynchronous generators. These subscriptions handle various scenarios, including client-side unsubscription and server-side abort signals.

• Router Configuration:

  • Added a new demoRouter to the appRouter to organize and manage the new subscription procedures.

• WebviewController Enhancements:
  This pull request to src/webviews/api/configuration/appRouter.ts and src/webviews/api/extension-server/WebviewController.ts introduces new subscription procedures and enhanced documentation. The most important changes are grouped by theme below:

New Features:

• Introduced demoBasicSubscription and demoComplexSubscription procedures to demonstrate how to stream data from the server to the client using asynchronous generators.

Enhancements:

• Added signal property to BaseRouterContext to support subscription cancellation.

Documentation:

• Improved documentation for WebviewController to clarify its usage with tRPC for handling requests from the webview.
• Added detailed comments and usage examples for new subscription procedures to guide developers on how to implement and use them.

Code Organization:

• Updated appRouter to include the new demoRouter for better modularity.
  These changes collectively enhance the functionality, error handling, and integration capabilities of the application, providing a more robust and flexible development environment.