Group Chat Overview - Autogen

@AutoGen.Core.IGroupChat is a fundamental feature in AutoGen. It provides a way to organize multiple agents under the same context and work together to resolve a given task.

In AutoGen, there are two types of group chat:

@AutoGen.Core.RoundRobinGroupChat : This group chat runs agents in a round-robin sequence. The chat history plus the most recent reply from the previous agent will be passed to the next agent.
@AutoGen.Core.GroupChat : This group chat provides a more dynamic yet controlable way to determine the next speaker agent. You can either use a llm agent as group admin, or use a @AutoGen.Core.Graph, which is introduced by this PR, or both to determine the next speaker agent.

[!NOTE] In @AutoGen.Core.GroupChat, when only the group admin is used to determine the next speaker agent, it's recommented to use a more powerful llm model, such as gpt-4 to ensure the best experience.