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/inference/fixtures.py

@@ -9,7 +9,7 @@ import os
 import pytest
 import pytest_asyncio
 
-from llama_stack.apis.models import Model
+from llama_stack.apis.models import ModelInput
 
 from llama_stack.distribution.datatypes import Api, Provider
 from llama_stack.providers.inline.inference.meta_reference import (
@@ -162,10 +162,8 @@ async def inference_stack(request, inference_model):
         {"inference": inference_fixture.providers},
         inference_fixture.provider_data,
         models=[
-            Model(
-                identifier=inference_model,
-                provider_resource_id=inference_model,
-                provider_id=inference_fixture.providers[0].provider_id,
+            ModelInput(
+                model_id=inference_model,
             )
         ],
     )