Resource oriented design for shields (#399)

* init

* working bedrock tests

* bedrock test for inference fixes

* use env vars for bedrock guardrail vars

* add register in meta reference

* use correct shield impl in meta ref

* dont add together fixture

* right naming

* minor updates

* improved registration flow

* address feedback

---------

Co-authored-by: Dinesh Yeduguru <dineshyv@fb.com>

llama_stack/providers/inline/agents/meta_reference/tests/test_chat_agent.py

@@ -80,7 +80,7 @@ class MockInferenceAPI:
 
 class MockSafetyAPI:
     async def run_shield(
-        self, shield_type: str, messages: List[Message]
+        self, shield_id: str, messages: List[Message]
     ) -> RunShieldResponse:
         return RunShieldResponse(violation=None)