Better scenario from-file support

[sjmonson]

Summary

Allows for scenarios to pass yaml or json formatted synthetic config to data. Automatically handles conversion from single data to list.

---
target: "http://vllm-deployment.kserve-e2e-perf.svc.cluster.local:8000/"
profile: concurrent
rate: [1,2,4,8]
max_seconds: 120
data:
  prompt_tokens_min: 10
  prompt_tokens_max: 8192
  prompt_tokens: 4096
  prompt_tokens_stdev: 2048
  output_tokens_min: 10
  output_tokens_max: 2048
  output_tokens: 1024
  output_tokens_stdev: 512

---

• "I certify that all code in this PR is my own, except as noted below."

Use of AI

• Includes AI-assisted code completion
• Includes code generated by an AI application
• Includes AI-generated tests (NOTE: AI written tests should have a docstring that includes ## WRITTEN BY AI ##)