.Net Agents - Design `AgentChat` serialization / deserialization

[crickman]

While agent related serialization has been identified and iscussed, it hasn't been formally addressed and expicitly deprioritized in previous effort.

The need exists to serialize and deserialize an agent conversation (AgentChat).

For OpenAI Assistant API, this occurs automatically. A chat-completion based agent would have its own seralization requirements.

In addition to the core conversation, the issue presents itself how to memoize and re-hydrate agent instances. For the OpenAI Assistant API, this can occur using the assistant identifier. Still, as with any agent type, there are initializatin aspects that are non-trivial to consider:

• Configuration: api-key, endpoint, model, etc...
• Plug-Ins: how to re-iniatialize