[Frontend] Support simpler image input format

[yue-anyscale]

Why we make this change:
Some users use a simpler image input format (example below), which we cannot handle properly now:

[
  {
    "role": "user",
    "content": [
      "what's the animals in the image?",
      {
        "image_url": "https://animals.com/dog"
      },
      {
        "image_url": "https://animals.com/cat"
      },
      
    ]
  }
]

OpenAI can handle this format well even though they didn't document in the API reference.

Changes:
Modified the input parsing logic to support more content types within the "content" field - string, simpler image_url, simpler audio_url.

[github-actions]

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[comaniac]

Overall LGTM. Also please add a unit test in tests/entrypoints/test_chat_utils.py

Also cc @ywang96 @DarkLight1337

[yue-anyscale]

Overall LGTM. Also please add a unit test in tests/entrypoints/test_chat_utils.py

Also cc @ywang96 @DarkLight1337

Thanks for review! I just addressed comments and added unittest.

[comaniac]

Overall LGTM. Please fix the type errors instead of ignoring them.
After that I'll put ready to unblock the rest CI tests.

[yue-anyscale]

Overall LGTM. Please fix the type errors instead of ignoring them. After that I'll put ready to unblock the rest CI tests.

Thanks for the reviews! Fixed.