feat: Add open benchmark template codegen (#1579)

## What does this PR do?

As title, add codegen for open-benchmark template

## test 

checked the new generated run.yaml file and it's identical before and
after the change

Also add small improvement to together template so that missing
TOGETHER_API_KEY won't crash the server which is the consistent user
experience as other remote providers

llama_stack/templates/together/run-with-safety.yaml

@@ -16,7 +16,7 @@ providers:
     provider_type: remote::together
     config:
       url: https://api.together.xyz/v1
-      api_key: ${env.TOGETHER_API_KEY}
+      api_key: ${env.TOGETHER_API_KEY:}
   - provider_id: sentence-transformers
     provider_type: inline::sentence-transformers
     config: {}

llama_stack/templates/together/run.yaml

@@ -16,7 +16,7 @@ providers:
     provider_type: remote::together
     config:
       url: https://api.together.xyz/v1
-      api_key: ${env.TOGETHER_API_KEY}
+      api_key: ${env.TOGETHER_API_KEY:}
   - provider_id: sentence-transformers
     provider_type: inline::sentence-transformers
     config: {}