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/providers/tests/scoring/fixtures.py

@@ -7,6 +7,8 @@
 import pytest
 import pytest_asyncio
 
+from llama_stack.apis.models import ModelInput
+
 from llama_stack.distribution.datatypes import Api, Provider
 
 from llama_stack.providers.tests.resolver import resolve_impls_for_test_v2
@@ -76,20 +78,14 @@ async def scoring_stack(request, inference_model):
         [Api.scoring, Api.datasetio, Api.inference],
         providers,
         provider_data,
-    )
-
-    provider_id = providers["inference"][0].provider_id
-    await impls[Api.models].register_model(
-        model_id=inference_model,
-        provider_id=provider_id,
-    )
-    await impls[Api.models].register_model(
-        model_id="Llama3.1-405B-Instruct",
-        provider_id=provider_id,
-    )
-    await impls[Api.models].register_model(
-        model_id="Llama3.1-8B-Instruct",
-        provider_id=provider_id,
+        models=[
+            ModelInput(model_id=model)
+            for model in [
+                inference_model,
+                "Llama3.1-405B-Instruct",
+                "Llama3.1-8B-Instruct",
+            ]
+        ],
     )
 
     return impls