Add swarm manager option

[yiranwu0]

Why are these changes needed?

Add Afterwork.SWARM_MANAGER option.

Related issue number

4 th point in Issue #193

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/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.

[marklysze]

Thanks @yiranwu0... the auto speaker selection will also look at all agents in the underlying groupchat, and we have the tool executor agent, nested chat agents, and the user proxy agent in there. I think we need to filter out the tool executor, what do you think?

[marklysze]

Thanks for the chat @yiranwu0, agree we can push forward without excluding agents and consider putting in a description on the tool executor to minimise the chance of it being selected.

[qingyun-wu]

@yiranwu0 could you fix the conflicts! Thanks!

[marklysze]

Just a note that I'm adding a system message to minimise the tool executor being called by auto speaker selection in PR #316:

    tool_execution = ConversableAgent(
        name=__TOOL_EXECUTOR_NAME__,
        system_message="Tool Execution, do not use this agent directly.",
    )

So this is good to go, thanks @yiranwu0!

[marklysze]

@yiranwu0, I just noticed that we lost the return of "auto" for SWARM_MANAGER, so I'll add an updated PR for it.