Make AgentChat Teams Declarative

[victordibia]

Why are these changes needed?

Support a developer experience where an AgentChat team configuration can be serialized to a declarative specification.

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

TBD

• Agent serialization is currently blocked on Declarative BaseChat Agents #5055 to enable agent serialization. Once done, will update this.
• Update tests

Related issue number

Closes #5064

Checks

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.20%. Comparing base (acb9117) to head (9d753af).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5071       +/-   ##
===========================================
+ Coverage   69.40%   83.20%   +13.80%     
===========================================
  Files         163       59      -104     
  Lines       10494     2698     -7796     
===========================================
- Hits         7283     2245     -5038     
+ Misses       3211      453     -2758     

Flag	Coverage Δ
unittests	83.20% <ø> (+13.80%)	⬆️

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