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/distribution/routers/inference.py

@@ -426,6 +426,7 @@ class InferenceRouter(Inference):
         user: str | None = None,
         guided_choice: list[str] | None = None,
         prompt_logprobs: int | None = None,
+        suffix: str | None = None,
     ) -> OpenAICompletion:
         logger.debug(
             f"InferenceRouter.openai_completion: {model=}, {stream=}, {prompt=}",
@@ -456,6 +457,7 @@ class InferenceRouter(Inference):
             user=user,
             guided_choice=guided_choice,
             prompt_logprobs=prompt_logprobs,
+            suffix=suffix,
         )
 
         provider = self.routing_table.get_provider_impl(model_obj.identifier)