feat: Add suffix to openai_completions (#2449)

For code completion apps need "fill in the middle" capabilities. 
Added option of `suffix` to `openai_completion` to enable this. 
Updated ollama provider to showcase the same. 

### Test Plan 
```
pytest -sv --stack-config="inference=ollama"  tests/integration/inference/test_openai_completion.py --text-model qwen2.5-coder:1.5b -k test_openai_completion_non_streaming_suffix
```

### OpenAI Sample script
```
from openai import OpenAI

client = OpenAI(base_url="http://localhost:8321/v1/openai/v1")

response = client.completions.create(
    model="qwen2.5-coder:1.5b",
    prompt="The capital of ",
    suffix="is Paris.",
    max_tokens=10,
)

print(response.choices[0].text)
``` 
### Output
```
France is ____.

To answer this question, we 
```

llama_stack/apis/inference/inference.py

@@ -1038,6 +1038,8 @@ class InferenceProvider(Protocol):
         # vLLM-specific parameters
         guided_choice: list[str] | None = None,
         prompt_logprobs: int | None = None,
+        # for fill-in-the-middle type completion
+        suffix: str | None = None,
     ) -> OpenAICompletion:
         """Generate an OpenAI-compatible completion for the given prompt using the specified model.
 
@@ -1058,6 +1060,7 @@ class InferenceProvider(Protocol):
         :param temperature: (Optional) The temperature to use.
         :param top_p: (Optional) The top p to use.
         :param user: (Optional) The user to use.
+        :param suffix: (Optional) The suffix that should be appended to the completion.
         :returns: An OpenAICompletion.
         """
         ...