[.Net] refactor over streaming version api

[LittleLittleCloud]

Why are these changes needed?

This PR creates a more clear and easy-to-follow pattern between the non-streaming and streaming api.

For non-streaming, we have the following definition

• IAgent: agent interface
• IMiddleware: middleware interface that extends an agent's behavior
• MiddlewareAgent and MiddlewareAgent<TAgent>: A built-in agent that implements IAgent and work with middleware

And corresponding streaming is below. Note that IStreamingAgent is also an IAgent and IStreamingMiddleware is also an IMiddleware.

• IStreamingAgent: IAgent + streaming api
• IStreamingMiddleware: IMiddleware + streaming api
• StreamingMiddlewareAgent and StreamingMiddlewareAgent<TAgent>: MiddlewareAgent | MiddlewareAgent<TAgent> + streaming api, which implements IStreamingAgent

And below is the relationship table around middleware, agent and MiddlewareAgent

register	IAgent	IStreamingAgent
IMiddleware	MiddlewareAgent<IAgent>	MiddlewareAgent<IStreamingAgent>
IStreamingMiddleware	MiddlewareAgent<IAgent>	MiddlewareStreamingAgent<IStreamingAgent>

Here's a bit explanation:

• IAgent + IMiddleware: MiddlewareAgent<Agent>
• IStreamingAgent + IStreamingMiddleware: StreamingMiddlewareAgent<StreamingAgent>
• IAgent + IStreamingMiddleware: This will be equivalent to IAgent + IMiddleware where IStreamingMiddleware be converted to IMiddleware
• IStreamingAgent + IMiddleware: This will be equivalent to IAgent + IMiddleware where IStreamingAgent be converted to IAgent

Note

Only IStreamingAgent + IStreamingMiddleware combination results to IStreamingAgent. For all other combination, the new agent will be IAgent and doesn't support streaming

Related issue number

• [.Net]: Change the return type of the streaming api in IStreamingAgent and IStreamingMiddleware from Task<IAsyncEnumerable<T>> to IAsyncEnumerable<T> #2470

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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