Multimodal message as a List: introducing AGImage

[BeibinLi]

Why are these changes needed?

This PR is part of Multimodal Orchestration: #1975

It will

1. This PR introduces a new way of composing messages to agents.

agent.send(recipient, message=["What is in the image?", AGImage("my/img/example.jpg")]
agent.send(recipient, message="Thanks! Can you write a poet for the above image?"]
agent.send(recipient, message=AGImage("http://another.com/example/example.png")

2. Disable HTML tag detection in message by default.
   • Enable HTML tag detection with one flag enable_mm_tag=True.
3. During human_input_mode, users can input message with HTML tag as syntax candy for end-users typing in terminal.
4. Introduce the AGImage class to declare images in message.

Related issue number

#2160 and #2026

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-commenter]

Codecov Report

Attention: Patch coverage is 36.51685% with 113 lines in your changes are missing coverage. Please review.

Project coverage is 49.02%. Comparing base (2053dd9) to head (9ec6a4b).

Files	Patch %	Lines
autogen/multimodal_utils.py	19.23%	62 Missing and 1 partial ⚠️
autogen/agentchat/contrib/img_utils.py	32.65%	32 Missing and 1 partial ⚠️
autogen/agentchat/conversable_agent.py	53.57%	9 Missing and 4 partials ⚠️
autogen/oai/client.py	76.92%	1 Missing and 2 partials ⚠️
autogen/oai/openai_utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2196       +/-   ##
===========================================
+ Coverage   37.94%   49.02%   +11.07%     
===========================================
  Files          77       78        +1     
  Lines        7784     7939      +155     
  Branches     1667     1855      +188     
===========================================
+ Hits         2954     3892      +938     
+ Misses       4580     3713      -867     
- Partials      250      334       +84     

Flag	Coverage Δ
unittest	14.29% <16.29%> (?)
unittests	47.97% <36.51%> (+10.04%)	⬆️

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