Add LLM benchmarking framework to staging

[kubraaksux]

Generic LLM benchmark suite for evaluating inference performance across different backends (vLLM, Ollama, OpenAI, MLX).

Features:

• Multiple workload categories: math (GSM8K), reasoning (BoolQ, LogiQA), summarization (XSum, CNN/DM), JSON extraction
• Pluggable backend architecture for different inference engines
• Performance metrics: latency, throughput, memory usage
• Accuracy evaluation per workload type
• HTML report generation

This framework can be used to evaluate SystemDS LLM inference components once they are developed.