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 
```

docs/_static/llama-stack-spec.html

@@ -12777,6 +12777,10 @@
                     },
                     "prompt_logprobs": {
                         "type": "integer"
+                    },
+                    "suffix": {
+                        "type": "string",
+                        "description": "(Optional) The suffix that should be appended to the completion."
                     }
                 },
                 "additionalProperties": false,

docs/_static/llama-stack-spec.yaml

@@ -8903,6 +8903,10 @@ components:
             type: string
         prompt_logprobs:
           type: integer
+        suffix:
+          type: string
+          description: >-
+            (Optional) The suffix that should be appended to the completion.
       additionalProperties: false
       required:
         - model