### What problem does this PR solve?

Added OpenAI-like completion api, related to #4672, #4705 

This function allows users to interact with a model to get responses
based on a series of messages.
If `stream` is set to True, the response will be streamed in chunks,
mimicking the OpenAI-style API.

#### Example usage:

```bash
curl -X POST https://ragflow_address.com/api/v1/chats_openai/<chat_id>/chat/completions \
    -H "Content-Type: application/json" \
    -H "Authorization: Bearer $RAGFLOW_API_KEY" \
    -d '{
        "model": "model",
        "messages": [{"role": "user", "content": "Say this is a test!"}],
        "stream": true
    }'
```

Alternatively, you can use Python's `OpenAI` client:

```python
from openai import OpenAI

model = "model"
client = OpenAI(api_key="ragflow-api-key", base_url=f"http://ragflow_address/api/v1/chats_openai/<chat_id>")

completion = client.chat.completions.create(
    model=model,
    messages=[
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": "Who you are?"},
        {"role": "assistant", "content": "I am an AI assistant named..."},
        {"role": "user", "content": "Can you tell me how to install neovim"},
    ],
    stream=True
)

stream = True
if stream:
    for chunk in completion:
        print(chunk)
else:
    print(completion.choices[0].message.content)
```
### Type of change
- [x] New Feature (non-breaking change which adds functionality)

### Related Issues
Related to #4672, #4705