Message "content" now supports both `str` and `List` in Agents (#713)

* Change "content" type in Conversable Agent

* content and system_message support str and List
Update for all other agents

* Content_str now also takes None as input

* Group Chat now works with LMM too

* Style: newline for import in Conversable Agentt

* Add test for gourpchat + lmm

* Resolve comments
1. Undo AssistantAgent changes
2. Modify the asserts and raises in `content_str` function and update
test accordingly.

* Undo AssistantAgent

* Update comments and add assertion for LMM

* Typo fix in docstring for content_str

* Remove “None” out conversable_agent.py

* Lint message to dict in multimodal_conversable_agent.py

* Address lint issues

* linting

* Move lmm test into contrib test

* Resolve 2 comments

* Move img_utils into contrib folder

* Resolve img_utils path issues