[.Net] Streaming support in IAgent, support for streaming middleware plus pulling out pretty-print, user-input and function-call as middleware for easier testing

[LittleLittleCloud]

Why are these changes needed?

This PR contains the following changes to the middleware pattern in AutoGen.Net

• Streaming support for IAgent. Agent can implement IStreamingAgent to declare itself as streaming-support. One example for IStreamingAgent is GPTAgent. (Plus test coverage)
• create IMiddleware and IStreamingMiddleware interfaces. These two interfaces will be the abstraction layer of all middlewares inside AutoGen.Net. The experience of registering middleware through lambda is reserved via DelegateMiddleware (for IMiddleware) and DelegateStreamingMiddleware (for IStreamingMiddleware)
• extract pretty print, user input and function call into separate middleware component so that it's eaiser to add test coverage plus consuming by external user.

For example, with HumanInputMiddleware, it's possible to turn any IAgent into a user proxy

IAgent gpt, assistant;
var humanInputMiddleware = new HumanInputMiddleware();
agent = agent.RegisterMiddleware(humanInputMiddleware);

await gpt.SendAsync(assistant, "hello");

// Please give feedback: Press enter or type 'exit' to stop the conversation.
// ...user input

Related issue number

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.