Allow direct payloads to be passed into GenAI-Perf

[dyastremsky]

Allow direct payloads to be passed into GenAI-Perf as a JSONL file.

If we proceed with this feature, we'll need to update the parser descriptions and docs. We'll want to add unit testing and a CI test.

Example:

Server run command: docker run -it --net=host --gpus=all vllm/vllm-openai:latest --model gpt2 --dtype float16 --max-model-len 1024

Direct payload file (JSONL):

{"model": "gpt2", "prompt": ["Generate an example of a user profile."], "stream": false, "temperature": 0.0}

Example command: genai-perf profile -v -m gpt2 --service-kind openai --endpoint-type completions --input-file payload:direct_payload_example.jsonl

Output: