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/remote/safety/bedrock/bedrock.py

@@ -20,11 +20,6 @@ from .config import BedrockSafetyConfig
 logger = logging.getLogger(__name__)
 
 
-BEDROCK_SUPPORTED_SHIELDS = [
-    ShieldType.generic_content_shield,
-]
-
-
 class BedrockSafetyAdapter(Safety, ShieldsProtocolPrivate):
     def __init__(self, config: BedrockSafetyConfig) -> None:
         self.config = config