Refactor agent chat to prepare for handoff/swarm

[ekzhu]

1. Add handoff message type to chat message types
2. Add Swarm group chat that uses handoff message to select next speaker

from autogen_agentchat.teams import Swarm

# agent1, agent2, agent3 must be able to generate `HandoffMessage`. 

swarm = Swarm([agent1, agent2, agent3])
result = await swarm.run("task")

3. Remove tool call and tool call result message types from chat message types
4. Remove BaseToolUseChatAgent, move tool call handling from group chat's chat agent container upward to the ToolUseAssistantAgent implementation, which subclasses BaseChatAgent directly.
5. Renaming for better clarity

TODO as next PRs:

1. Implement mechanism inside a chat agent to generate handoff message
2. Consider merging CodingAssistantAgent and ToolUseAssistantAgent into a single agent class AssistantAgent, which also has the ability to generate handoff messages.
3. Rename registered_tools to tools.
4. Should we consider chat agent returning a sequence of chat messages rather than a single one -- what is the usage case?

Partially resolves - #3946 and #3864

[afourney]

Oooh, I do like that top-level interface:

swarm = Swarm([agent1, agent2, agent3])
result = await swarm.run("task")

[victordibia]

+1 on the high level swarm interface. At some point, it might be good to have a naming convention for everything inside autogen_agentchat.teams. Right now we have things like teams.GroupChat, teams.Swarm ... perhaps something like teams.GroupChatTeam and teams.SwarmTeam to anchor users that both are types of teams.

+1 again on unifying CodingAssistantAgent and ToolUseAssistantAgent