dotnet sample project to implement Agent Server using Google A2A, MCP, and Semantic Kernel

## Description
Based on the current progress we observe from Google A2A community, it could be good to also support .NET client sample and SDK for A2A server, which connects to an agent running with MCP and Semantic Kernel.

## Clarification
[Semantic Kernel](https://github.com/microsoft/semantic-kernel) is an AI agent framework which connects the .NET software functionalities to be directly callable from LLM models. Combination of Semantic Kernel and MCP would create a very powerful tool for agent programming which can also act as a MCP client.

## Goal
Based on the clarification section and description, it is necessary to provide a complete example of how to create an AI agent from scratch which is powered by MCP and Semantic Kernel and follows Google A2A protocol. This will give us a more clear vision of creating a full AI agent which can communicate with other agents built in C# and .NET using both very popular frameworks in .NET which are Semantic Kernel and MCP.

## Related PRs
google-a2a/A2A#425

https://github.com/user-attachments/assets/09550440-727b-4eab-a6ce-a6938674c835

The issue is to create a .NET sample project implementing an Agent Server using Google A2A, MCP, and Semantic Kernel. Start by reviewing the linked PR google-a2a/A2A#425 and the provided asset to understand the A2A protocol and existing integration patterns. Examine the a2a-samples repository for existing sample structures. The goal is a complete example from scratch, so first understand how Semantic Kernel and MCP connect, then design the agent server architecture. 'Done' is a working sample project demonstrating the full integration.

由索引模型根據 Issue 內容生成。