support using deepseek models

[mattguo]

I'd like to use deepseek models (using the same OpenAI API protocol, much cheaper than gtp4 and much stronger than gpt3) together with this plugin.

My usage:`
in the plugin settings, set

• openai api key to deepseek api key, which looks like: sk-123456....
• endpoint to https://api.deepseek.com
• Completion engine to deepseek-chat, or deepseek-reasoner
• (Optional) clear up OpenAI Chat Prompt in settings
  Then it worked for me locally for features like summarizing text.

Before this change it couldn't work due to the HTTPS request send the prompt in body.prompt, rather than in the messages payload.

Btw, its API doc: https://api-docs.deepseek.com/

[mattguo]

Oh, I just found #132 is better than this one. I will drop this pull request.