Merge SwarmAgent into ConversableAgent

[marklysze]

Why are these changes needed?

To expand the use of swarms to allow all ConversableAgent-based agents, this merges SwarmAgent into ConversableAgent.

Progress:

• Move SwarmAgent-specific init parameters to ConversableAgent
• Move SwarmAgent initialisation functionality into the a/initiate_swarm_chat functions
• Move swarm-specific functions under SwarmAgent out, like register_hand_off, _update_conditional_functions, generate_swarm_tool_reply
• Move/Merge nested chat with carryover into ConversableAgent (process_nested_chat_carryover, _summary_from_nested_chats)
• Update a/initiate_swarm_chat, AFTER_WORK, ON_CONDITION, SwarmResult, to use ConversableAgent instead of SwarmAgent
• Move SwarmAgent tests to ConversableAgent tests
• Update documentation /notebooks referring to SwarmAgent to ConversableAgent
• Update documentation changing agent.register_hand_off to register_hand_off(agent
• Add notes to YouTube videos that SwarmAgent is now merged into ConversableAgent
• Finalise interface via discussion

QUESTIONS/NOTES FOR REVIEWER:

• SwarmAgents could take a list of functions to register (e.g. my_agent = SwarmAgent(functions=[function_a, function_b])). These functions were registered for LLM but not for execution (as a separate agent would execute). How should this work on ConversableAgent? Should it be the same or should there be a way to specify LLM and/or Execution for each function? I have left it as the same for now.
• In order to show the agent name for a transition function, I have implemented __str__ on ConversableAgent and the associated internal function (_get_display_name()) within it can be overwritten on a ConversableAgent instance, which I've done for any agents associated with a swarm. This means that if you do str(my_conversable_agent) you will get the name of the agent rather than the type as a string.

MIGRATION INSTRUCTIONS:

• Change SwarmAgent to ConversableAgent
• Instead of agent.register_hand_off, import and use register_hand_off(agent=abc, ...)
• UPDATE_SYSTEM_MESSAGE is now UpdateSystemMessage (deprecation warning added)
• ON_CONDITION is now OnCondition (deprecation warning added)
• AFTER_WORK is now AfterWork (deprecation warning added)
• ... TBD

Related issue number

#26

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/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.