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/models/models.py

@@ -7,20 +7,38 @@
 from typing import Any, Dict, List, Literal, Optional, Protocol, runtime_checkable
 
 from llama_models.schema_utils import json_schema_type, webmethod
-from pydantic import Field
+from pydantic import BaseModel, Field
 
 from llama_stack.apis.resource import Resource, ResourceType
 
 
-@json_schema_type
-class Model(Resource):
-    type: Literal[ResourceType.model.value] = ResourceType.model.value
+class CommonModelFields(BaseModel):
     metadata: Dict[str, Any] = Field(
         default_factory=dict,
         description="Any additional metadata for this model",
     )
 
 
+@json_schema_type
+class Model(CommonModelFields, Resource):
+    type: Literal[ResourceType.model.value] = ResourceType.model.value
+
+    @property
+    def model_id(self) -> str:
+        return self.identifier
+
+    @property
+    def provider_model_id(self) -> str:
+        return self.provider_resource_id
+
+
+@json_schema_type
+class ModelInput(CommonModelFields):
+    model_id: str
+    provider_id: Optional[str] = None
+    provider_model_id: Optional[str] = None
+
+
 @runtime_checkable
 class Models(Protocol):
     @webmethod(route="/models/list", method="GET")