[Evals API][6/n] meta-reference llm as judge, registration for ScoringFnDefs (#330)

* wip scoring refactor

* llm as judge, move folders

* test full generation + eval

* extract score regex to llm context

* remove prints, cleanup braintrust in this branch

* change json -> class

* remove initialize

* address nits

* check identifier prefix

* udpate MANIFEST

llama_stack/providers/tests/eval/provider_config_example.yaml

@@ -16,3 +16,7 @@ providers:
       provider_type: remote::tgi
       config:
         url: http://127.0.0.1:5009
+    - provider_id: test-tgi-2
+      provider_type: remote::tgi
+      config:
+        url: http://127.0.0.1:5010