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/inline/safety/llama_guard/llama_guard.py

@@ -128,7 +128,6 @@ class LlamaGuardSafetyImpl(Safety, ShieldsProtocolPrivate):
         pass
 
     async def register_shield(self, shield: Shield) -> None:
-        print(f"Registering shield {shield}")
         if shield.shield_type != ShieldType.llama_guard:
             raise ValueError(f"Unsupported shield type: {shield.shield_type}")