Enable sane naming of registered objects with defaults (#429)

# What does this PR do? 

This is a follow-up to #425. That PR allows for specifying models in the
registry, but each entry needs to look like:

```yaml
- identifier: ...
  provider_id: ...
  provider_resource_identifier: ...
```

This is headache-inducing.

The current PR makes this situation better by adopting the shape of our
APIs. Namely, we need the user to only specify `model-id`. The rest
should be optional and figured out by the Stack. You can always override
it.

Here's what example `ollama` "full stack" registry looks like (we still
need to kill or simplify shield_type crap):
```yaml
models:
- model_id: Llama3.2-3B-Instruct
- model_id: Llama-Guard-3-1B
shields:
- shield_id: llama_guard
  shield_type: llama_guard
```

## Test Plan

See test plan for #425. Re-ran it.

llama_stack/apis/scoring_functions/scoring_functions.py

@@ -66,11 +66,7 @@ ScoringFnParams = Annotated[
 ]
 
 
-@json_schema_type
-class ScoringFn(Resource):
-    type: Literal[ResourceType.scoring_function.value] = (
-        ResourceType.scoring_function.value
-    )
+class CommonScoringFnFields(BaseModel):
     description: Optional[str] = None
     metadata: Dict[str, Any] = Field(
         default_factory=dict,
@@ -85,6 +81,28 @@ class ScoringFn(Resource):
     )
 
 
+@json_schema_type
+class ScoringFn(CommonScoringFnFields, Resource):
+    type: Literal[ResourceType.scoring_function.value] = (
+        ResourceType.scoring_function.value
+    )
+
+    @property
+    def scoring_fn_id(self) -> str:
+        return self.identifier
+
+    @property
+    def provider_scoring_fn_id(self) -> str:
+        return self.provider_resource_id
+
+
+@json_schema_type
+class ScoringFnInput(CommonScoringFnFields, BaseModel):
+    scoring_fn_id: str
+    provider_id: Optional[str] = None
+    provider_scoring_fn_id: Optional[str] = None
+
+
 @runtime_checkable
 class ScoringFunctions(Protocol):
     @webmethod(route="/scoring_functions/list", method="GET")