[Benchmark] Refactor perf script to use benchmark cli (#1524)

### What this PR does / why we need it?

Since, `vllm bench` cli has optimized enough for production use(support
more datasets), we are now do not need to copy vllm codes, now , with
vllm installed, we can easily use the benchmark cli
### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
CI passed

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

benchmarks/tests/serving-tests.json

@@ -18,7 +18,7 @@
     },
     "client_parameters": {
       "model": "Qwen/Qwen2.5-VL-7B-Instruct",
-      "backend": "openai-chat",
+      "endpoint_type": "openai-chat",
       "dataset_name": "hf",
       "hf_split": "train",
       "endpoint": "/v1/chat/completions",
@@ -44,7 +44,7 @@
     },
     "client_parameters": {
       "model": "Qwen/Qwen3-8B",
-      "backend": "vllm",
+      "endpoint_type": "vllm",
       "dataset_name": "sharegpt",
       "dataset_path": "/github/home/.cache/datasets/ShareGPT_V3_unfiltered_cleaned_split.json",
       "num_prompts": 200
@@ -68,7 +68,7 @@
     },
     "client_parameters": {
       "model": "Qwen/Qwen2.5-7B-Instruct",
-      "backend": "vllm",
+      "endpoint_type": "vllm",
       "dataset_name": "sharegpt",
       "dataset_path": "/github/home/.cache/datasets/ShareGPT_V3_unfiltered_cleaned_split.json",
       "num_prompts": 200