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/agents/fixtures.py

@@ -9,7 +9,7 @@ import tempfile
 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.agents.meta_reference import (
@@ -71,13 +71,9 @@ async def agents_stack(request, inference_model, safety_model):
         if fixture.provider_data:
             provider_data.update(fixture.provider_data)
 
-    inf_provider_id = providers["inference"][0].provider_id
-    safety_provider_id = providers["safety"][0].provider_id
-
-    shield = get_shield_to_register(
-        providers["safety"][0].provider_type, safety_provider_id, safety_model
+    shield_input = get_shield_to_register(
+        providers["safety"][0].provider_type, safety_model
     )
-
     inference_models = (
         inference_model if isinstance(inference_model, list) else [inference_model]
     )
@@ -86,13 +82,11 @@ async def agents_stack(request, inference_model, safety_model):
         providers,
         provider_data,
         models=[
-            Model(
-                identifier=model,
-                provider_id=inf_provider_id,
-                provider_resource_id=model,
+            ModelInput(
+                model_id=model,
             )
             for model in inference_models
         ],
-        shields=[shield],
+        shields=[shield_input],
     )
     return impls[Api.agents], impls[Api.memory]