.Net Agents - AgentChat Serialization

[crickman]

Motivation and Context

Support ability to capture and restore AgentChat history:

https://github.com/microsoft/semantic-kernel/blob/main/docs/decisions/0048-agent-chat-serialization.md

Description

Introduces AgentChatSerializer that supports serialization and deserialization of entire conversation state.

{
  "Participants": [
    {
      "Id": "asst_YaLY1TRmxsIReVa0BiuhEDAj",
      "Name": "agent-1",
      "Type": "Microsoft.SemanticKernel.Agents.OpenAI.OpenAIAssistantAgent"
    },
    {
      "Id": "1e884890-c344-4bc0-9367-068c482a499d",
      "Name": "agent-2",
      "Type": "Microsoft.SemanticKernel.Agents.ChatCompletionAgent"
    }
  ],
  "History": [{"Role":{"Label":"user"},"Items":[{"$type":"TextContent","Text":"..."}],"ModelId":"gpt-35-turbo-16k"},{"Role":{"Label":"assistant"},"Items":[{"$type":"TextContent","Text":"..."}],"ModelId":"gpt-35-turbo-16k"}],
  "Channels": [
    {
      "ChannelKey": "4kjDzpCpeOLUNbsSUisHd9cphSzEAb6Hxdnmr\u002Bem1Jw=",
      "ChannelType": "Microsoft.SemanticKernel.Agents.OpenAI.OpenAIAssistantChannel",
      "ChannelState": "thread_ZSF1ovTVzyYy8cg9GEwmDWgy"
    },
    {
      "ChannelKey": "Vdx37EnWT9BS\u002BkkCkEgFCg9uHvHNw1\u002BhXMA4sgNMKs4=",
      "ChannelType": "Microsoft.SemanticKernel.Agents.ChatHistoryChannel",
      "ChannelState": [{"Role":{"Label":"user"},"Items":[{"$type":"TextContent","Text":"..."}],"ModelId":"gpt-35-turbo-16k"},{"Role":{"Label":"assistant"},"Items":[{"$type":"TextContent","Text":"..."}],"ModelId":"gpt-35-turbo-16k"}]
    }
  ]
}

Includes sample showing serialization and deserialization with ChatCompletionAgent and OpenAIAssistantAgent partipating on the same chat, as well as a chat that includes ChatCompletionAgent that calls a plug-in.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄