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/resource.py

@@ -17,14 +17,12 @@ class ResourceType(Enum):
     memory_bank = "memory_bank"
     dataset = "dataset"
     scoring_function = "scoring_function"
+    eval_task = "eval_task"
 
 
 class Resource(BaseModel):
     """Base class for all Llama Stack resources"""
 
-    # TODO: I think we need to move these into the child classes
-    # and make them `model_id`, `shield_id`, etc. because otherwise
-    # the config file has these confusing generic names in there
     identifier: str = Field(
         description="Unique identifier for this resource in llama stack"
     )