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.

docs/resources/llama-stack-spec.yaml

@@ -2068,7 +2068,7 @@ components:
           - $ref: '#/components/schemas/GraphMemoryBankParams'
         provider_id:
           type: string
-        provider_memorybank_id:
+        provider_memory_bank_id:
           type: string
       required:
       - memory_bank_id
@@ -2710,7 +2710,6 @@ components:
       - provider_id
       - type
       - shield_type
-      - params
       title: A safety shield resource that can be used to check content
       type: object
     ShieldCallStep:
@@ -3398,7 +3397,7 @@ info:
   description: "This is the specification of the llama stack that provides\n     \
     \           a set of endpoints and their corresponding interfaces that are tailored\
     \ to\n                best leverage Llama Models. The specification is still in\
-    \ draft and subject to change.\n                Generated at 2024-11-11 18:44:30.967321"
+    \ draft and subject to change.\n                Generated at 2024-11-12 11:16:58.657871"
   title: '[DRAFT] Llama Stack Specification'
   version: 0.0.1
 jsonSchemaDialect: https://json-schema.org/draft/2020-12/schema
@@ -4762,24 +4761,24 @@ security:
 servers:
 - url: http://any-hosted-llama-stack.com
 tags:
-- name: Datasets
-- name: Telemetry
-- name: PostTraining
-- name: MemoryBanks
-- name: Eval
-- name: Memory
-- name: EvalTasks
-- name: Models
-- name: Scoring
 - name: Inference
-- name: Shields
-- name: DatasetIO
-- name: Safety
 - name: Agents
-- name: SyntheticDataGeneration
-- name: ScoringFunctions
-- name: BatchInference
+- name: Telemetry
+- name: Eval
+- name: Models
 - name: Inspect
+- name: EvalTasks
+- name: ScoringFunctions
+- name: Memory
+- name: Safety
+- name: DatasetIO
+- name: MemoryBanks
+- name: Shields
+- name: PostTraining
+- name: Datasets
+- name: Scoring
+- name: SyntheticDataGeneration
+- name: BatchInference
 - description: <SchemaDefinition schemaRef="#/components/schemas/BuiltinTool" />
   name: BuiltinTool
 - description: <SchemaDefinition schemaRef="#/components/schemas/CompletionMessage"