Swarm Asynchronous initiate_swarm_chat

[marklysze]

Why are these changes needed?

Add a_initiate_swarm_chat for asynchronous swarm chat.

Includes considerable internal refactoring of initiate_swarm_chat into separate functions.

Includes a fix for GroupChat's a_run_chat to handle NoEligibleSpeaker exceptions.

Related issue number

Closes #299

Checks

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[yiranwu0]

Nice refactoring! Look good to me.