Optimize the server mode discovery workflow

[MarcoRossignoli]

This pull request includes several changes to the Playground project and the Microsoft.Testing.Platform components, focusing on project configuration, code cleanup, and feature enhancements. The most important changes include updates to project references, import statements, and the addition of new capabilities to the testing platform.

Project Configuration Updates:

• Consolidated ProjectReference tags in samples/Playground/Playground.csproj for better readability.
• Added an import statement for Microsoft.Testing.Platform.props in samples/Playground/Playground.csproj to include platform capabilities.

Code Cleanup:

• Uncommented previously commented-out debugging and test host controller code in samples/Playground/Program.cs to enable these features.

Feature Enhancements:

• Added a new boolean parameter isJsonRpcProtocol to several classes and methods to support JSON-RPC protocol in the test framework. [1] [2] [3] [4] [5]
• Introduced new project capabilities in src/Platform/Microsoft.Testing.Platform/buildMultiTargeting/Microsoft.Testing.Platform.props to handle test process exit and list-tests options in server mode.