feat: tool outputs metadata (#1155)

Summary:

Allows tools to output metadata. This is useful for evaluating tool
outputs, e.g. RAG tool will output document IDs, which can be used to
score recall.

Will need to make a similar change on the client side to support
ClientTool outputting metadata.

Test Plan:

LLAMA_STACK_CONFIG=fireworks pytest -s -v
tests/client-sdk/agents/test_agents.py

llama_stack/apis/tools/tools.py

@@ -72,6 +72,7 @@ class ToolInvocationResult(BaseModel):
     content: InterleavedContent
     error_message: Optional[str] = None
     error_code: Optional[int] = None
+    metadata: Optional[Dict[str, Any]] = None
 
 
 class ToolStore(Protocol):