Pass context between AssistantAgent for handoffs

[ekzhu]

Resolves #5045

Now AssistantAgent can pass context about tool calls happened in the same turn to another AssistantAgent through HandoffMessage

[ekzhu]

@jochenvw @vincent-herlemont please try this branch to verify if your issue is fixed.

[codecov]

Codecov Report

Attention: Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.95%. Comparing base (0363388) to head (dc0947c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/src/autogen_agentchat/agents/_assistant_agent.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5084      +/-   ##
==========================================
+ Coverage   69.91%   69.95%   +0.04%     
==========================================
  Files         168      168              
  Lines       10698    10716      +18     
==========================================
+ Hits         7479     7496      +17     
- Misses       3219     3220       +1     

Flag	Coverage Δ
unittests	69.95% <97.22%> (+0.04%)	⬆️

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