feat(server): add optional GPU metrics collection during prove

[Andrurachi]

closes #265

Adds optional hardware metrics collection during GPU proving operations.

Implementation Details:

• Adds --collect-gpu-metrics and --gpu-metrics-dir flags.
• Uses the ERE_GPU_METRICS env var to allow overriding the default nvidia-smi query fields.
• Wraps nvidia-smi using std::process::Command to write a timestamped CSV during zkvm.prove().
• ImplementsDrop to guarantee the nvidia-smi subprocess is killed and reaped.
• Catches NotFound errors; if nvidia-smi is missing, it logs a warning and continues proving without interrupting the server.

Testing Note:
Due to local CUDA architecture limitations, I am unable to verify the CSV output. Ready for verification on a capable machine :)