feat: [New Eval Benchamark] IfEval (#1708)

# What does this PR do?
In this PR, we added a new eval open benchmark IfEval based on paper
https://arxiv.org/abs/2311.07911 to measure the model capability of
instruction following.


## Test Plan
spin up a llama stack server with open-benchmark template

run `llama-stack-client --endpoint xxx eval run-benchmark
"meta-reference-ifeval" --model-id "meta-llama/Llama-3.3-70B-Instruct"
--output-dir "/home/markchen1015/" --num-examples 20` on client side and
get the eval aggregate results

llama_stack/apis/scoring_functions/scoring_functions.py

@@ -36,6 +36,7 @@ class ScoringFnParamsType(Enum):
 @json_schema_type
 class AggregationFunctionType(Enum):
     average = "average"
+    weighted_average = "weighted_average"
     median = "median"
     categorical_count = "categorical_count"
     accuracy = "accuracy"