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

docs/_static/llama-stack-spec.html

@@ -6268,6 +6268,7 @@
                 "type": "string",
                 "enum": [
                     "average",
+                    "weighted_average",
                     "median",
                     "categorical_count",
                     "accuracy"