Add support for reasoning_content field in BaseOpenAIChatCompletionClient

[hechty]

Currently, the OpenAI client in Autogen doesn't support the reasoning_content field, which is available in some models like Deepseek R1. This field is used to output the model's reasoning process. eg. The QwQ-32B in aliyun use this field for thought.

[ekzhu]

Could you help with this? You can submit a PR to fix OpenAIChatCompletionClient to handle the additional field.