Declarative BaseChat Agents

[victordibia]

Why are these changes needed?

Makes the following components declarative

• AssistantAgent
• UserProxyAgent
• ChatCompletionContext and child classes (bufffered, unbuffered, headtail)

Open issues (possibly future PR)

• Tools are not serialized yet (can be revised after Make FunctionTools Serializable (Declarative) #5052)
• UserProxy input_func is currently set to None
• Other agents (e.g., CodeExecutorAgent, SOMAgent, Agents in Ext) are not yet declarative.

Related issue number

Closes #5032

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

Attention: Patch coverage is 90.80460% with 8 lines in your changes missing coverage. Please review.

Project coverage is 73.11%. Comparing base (1f22a7b) to head (8696814).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/autogen_agentchat/agents/_user_proxy_agent.py	87.50%	2 Missing ⚠️
...model_context/_buffered_chat_completion_context.py	85.71%	2 Missing ⚠️
..._context/_head_and_tail_chat_completion_context.py	86.66%	2 Missing ⚠️
...odel_context/_unbounded_chat_completion_context.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5055      +/-   ##
==========================================
+ Coverage   72.95%   73.11%   +0.15%     
==========================================
  Files         116      116              
  Lines        6800     6869      +69     
==========================================
+ Hits         4961     5022      +61     
- Misses       1839     1847       +8     

Flag	Coverage Δ
unittests	73.11% <90.80%> (+0.15%)	⬆️

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