add braintrust to experimental-post-training template (#763)

as title, add braintrust to experimental-post-training template to run
llm as judge based eval for a finetuned model

llama_stack/templates/experimental-post-training/build.yaml

@@ -10,6 +10,7 @@ distribution_spec:
     - inline::meta-reference
     scoring:
     - inline::basic
+    - inline::braintrust
     post_training:
     - inline::torchtune
     datasetio:

llama_stack/templates/experimental-post-training/run.yaml

@@ -29,6 +29,10 @@ providers:
   - provider_id: basic
     provider_type: inline::basic
     config: {}
+  - provider_id: braintrust
+    provider_type: inline::braintrust
+    config:
+      openai_api_key: ${env.OPENAI_API_KEY:}
   datasetio:
   - provider_id: huggingface-0
     provider_type: remote::huggingface