Remove the "ShieldType" concept (#430)

# What does this PR do?

This PR kills the notion of "ShieldType". The impetus for this is the
realization:

> Why is keyword llama-guard appearing so many times everywhere,
sometimes with hyphens, sometimes with underscores?

Now that we have a notion of "provider specific resource identifiers"
and "user specific aliases" for those and the fact that this works with
models ("Llama3.1-8B-Instruct" <> "fireworks/llama-3pv1-..."), we can
follow the same rules for Shields.

So each Safety provider can make up a notion of identifiers it has
registered. This already happens with Bedrock correctly. We just
generalize it for Llama Guard, Prompt Guard, etc.

For Llama Guard, we further simplify by just adopting the underlying
model name itself as the identifier! No confusion necessary.

While doing this, I noticed a bug in our DistributionRegistry where we
weren't scoping identifiers by type. Fixed.

## Feature/Issue validation/testing/test plan

Ran (inference, safety, memory, agents) tests with ollama and fireworks
providers.

llama_stack/providers/tests/safety/fixtures.py

@@ -9,7 +9,7 @@ import pytest_asyncio
 
 from llama_stack.apis.models import ModelInput
 
-from llama_stack.apis.shields import ShieldInput, ShieldType
+from llama_stack.apis.shields import ShieldInput
 
 from llama_stack.distribution.datatypes import Api, Provider
 from llama_stack.providers.inline.safety.llama_guard import LlamaGuardConfig
@@ -41,7 +41,7 @@ def safety_llama_guard(safety_model) -> ProviderFixture:
             Provider(
                 provider_id="inline::llama-guard",
                 provider_type="inline::llama-guard",
-                config=LlamaGuardConfig(model=safety_model).model_dump(),
+                config=LlamaGuardConfig().model_dump(),
             )
         ],
     )
@@ -101,6 +101,8 @@ async def safety_stack(inference_model, safety_model, request):
     shield_provider_type = safety_fixture.providers[0].provider_type
     shield_input = get_shield_to_register(shield_provider_type, safety_model)
 
+    print(f"inference_model: {inference_model}")
+    print(f"shield_input = {shield_input}")
     impls = await resolve_impls_for_test_v2(
         [Api.safety, Api.shields, Api.inference],
         providers,
@@ -114,20 +116,14 @@ async def safety_stack(inference_model, safety_model, request):
 
 
 def get_shield_to_register(provider_type: str, safety_model: str) -> ShieldInput:
-    shield_config = {}
-    shield_type = ShieldType.llama_guard
-    identifier = "llama_guard"
-    if provider_type == "meta-reference":
-        shield_config["model"] = safety_model
-    elif provider_type == "remote::together":
-        shield_config["model"] = safety_model
-    elif provider_type == "remote::bedrock":
+    if provider_type == "remote::bedrock":
         identifier = get_env_or_fail("BEDROCK_GUARDRAIL_IDENTIFIER")
-        shield_config["guardrailVersion"] = get_env_or_fail("BEDROCK_GUARDRAIL_VERSION")
-        shield_type = ShieldType.generic_content_shield
+        params = {"guardrailVersion": get_env_or_fail("BEDROCK_GUARDRAIL_VERSION")}
+    else:
+        params = {}
+        identifier = safety_model
 
     return ShieldInput(
         shield_id=identifier,
-        shield_type=shield_type,
-        params=shield_config,
+        params=params,
     )

llama_stack/providers/tests/safety/test_safety.py

@@ -34,7 +34,6 @@ class TestSafety:
 
         for shield in response:
             assert isinstance(shield, Shield)
-            assert shield.shield_type in [v for v in ShieldType]
 
     @pytest.mark.asyncio
     async def test_run_shield(self, safety_stack):