Support declarative configs for BaseChatAgents (and derivatives) - UserProxy, AssistantAgent, WebSurferAgent

[victordibia]

Support a workflow where an Agent's configuration can be made declarative

# dump
agent = AssistantAgent(
    name="assistant",
    model_client=model_client,
    handoffs=["flights_refunder", "user"],
    tools=[web_search],
    system_message="Use tools to solve tasks.",
)
agent_config = agent.dump_component() 

{'name': 'assistant',
 'model_client': {'provider': 'autogen_ext.models.openai.OpenAIChatCompletionClient',
  'component_type': 'model',
  'version': 1,
  'component_version': 1,
  'config': {'model': 'gpt-4o'}},
 'tools': [],
 'handoffs': [{'target': 'flights_refunder',
   'description': 'Handoff to flights_refunder.',
   'name': 'transfer_to_flights_refunder',
   'message': 'Transferred to flights_refunder, adopting the role of flights_refunder immediately.'},
  {'target': 'user',
   'description': 'Handoff to user.',
   'name': 'transfer_to_user',
   'message': 'Transferred to user, adopting the role of user immediately.'}],
 'model_context': {'provider': 'autogen_core.model_context.UnboundedChatCompletionContext',
  'component_type': 'chat_completion_context',
  'version': 1,
  'component_version': 1,
  'config': {}},
 'description': 'An agent that provides assistance with ability to use tools.',
 'system_message': 'Use tools to solve tasks.',
 'reflect_on_tool_use': False,
 'tool_call_summary_format': '{result}'}

# load 
reloaded_agent = AssistantAgent.load_component(agent_config)