feat: Add rerank models and rerank API change (#3831)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
- Extend the model type to include rerank models.
- Implement `rerank()` method in inference router.
- Add `rerank_model_list` to `OpenAIMixin` to enable providers to
register and identify rerank models
- Update documentation.

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
```
pytest tests/unit/providers/utils/inference/test_openai_mixin.py
```

llama_stack/providers/utils/inference/openai_mixin.py

@@ -48,6 +48,7 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
     - overwrite_completion_id: If True, overwrites the 'id' field in OpenAI responses
     - download_images: If True, downloads images and converts to base64 for providers that require it
     - embedding_model_metadata: A dictionary mapping model IDs to their embedding metadata
+    - construct_model_from_identifier: Method to construct a Model instance corresponding to the given identifier
     - provider_data_api_key_field: Optional field name in provider data to look for API key
     - list_provider_model_ids: Method to list available models from the provider
     - get_extra_client_params: Method to provide extra parameters to the AsyncOpenAI client
@@ -121,6 +122,30 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
         """
         return {}
 
+    def construct_model_from_identifier(self, identifier: str) -> Model:
+        """
+        Construct a Model instance corresponding to the given identifier
+
+        Child classes can override this to customize model typing/metadata.
+
+        :param identifier: The provider's model identifier
+        :return: A Model instance
+        """
+        if metadata := self.embedding_model_metadata.get(identifier):
+            return Model(
+                provider_id=self.__provider_id__,  # type: ignore[attr-defined]
+                provider_resource_id=identifier,
+                identifier=identifier,
+                model_type=ModelType.embedding,
+                metadata=metadata,
+            )
+        return Model(
+            provider_id=self.__provider_id__,  # type: ignore[attr-defined]
+            provider_resource_id=identifier,
+            identifier=identifier,
+            model_type=ModelType.llm,
+        )
+
     async def list_provider_model_ids(self) -> Iterable[str]:
         """
         List available models from the provider.
@@ -416,21 +441,7 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
             if self.allowed_models and provider_model_id not in self.allowed_models:
                 logger.info(f"Skipping model {provider_model_id} as it is not in the allowed models list")
                 continue
-            if metadata := self.embedding_model_metadata.get(provider_model_id):
-                model = Model(
-                    provider_id=self.__provider_id__,  # type: ignore[attr-defined]
-                    provider_resource_id=provider_model_id,
-                    identifier=provider_model_id,
-                    model_type=ModelType.embedding,
-                    metadata=metadata,
-                )
-            else:
-                model = Model(
-                    provider_id=self.__provider_id__,  # type: ignore[attr-defined]
-                    provider_resource_id=provider_model_id,
-                    identifier=provider_model_id,
-                    model_type=ModelType.llm,
-                )
+            model = self.construct_model_from_identifier(provider_model_id)
             self._model_cache[provider_model_id] = model
 
         return list(self._model_cache.values())