WIP: Multiturn Benchmarking Support

[sjmonson]

PR Status

• Rework scheduler to unify RPS and concurrent code pathways
• Support scheduling request multi-turn conversions where each request feeds into the next
• Support generating synthetic multi-turn conversations
• Support parsing external multi-turn datasets
• Add "users" variant of concurrent; where each concurrent thread handles a single multi-turn conversion at a time

Closes #199

[nm-red-hat-upstream-automation-bot]

📦 Build Artifacts Available
The build artifacts (.whl and .tar.gz) have been successfully generated and are available for download: https://github.com/neuralmagic/guidellm/actions/runs/15934063548/artifacts/3421762392.
They will be retained for up to 30 days.

[nm-red-hat-upstream-automation-bot]

📦 Build Artifacts Available
The build artifacts (.whl and .tar.gz) have been successfully generated and are available for download: https://github.com/neuralmagic/guidellm/actions/runs/15934227250/artifacts/3421819489.
They will be retained for up to 30 days.

[nm-red-hat-upstream-automation-bot]

📦 Build Artifacts Available
The build artifacts (.whl and .tar.gz) have been successfully generated and are available for download: https://github.com/neuralmagic/guidellm/actions/runs/15935134867/artifacts/3422151072.
They will be retained for up to 30 days.

[nm-red-hat-upstream-automation-bot]

📦 Build Artifacts Available
The build artifacts (.whl and .tar.gz) have been successfully generated and are available for download: https://github.com/neuralmagic/guidellm/actions/runs/15935412052/artifacts/3422245922.
They will be retained for up to 30 days.

[nm-red-hat-upstream-automation-bot]

📦 Build Artifacts Available
The build artifacts (.whl and .tar.gz) have been successfully generated and are available for download: https://github.com/neuralmagic/guidellm/actions/runs/16201694709/artifacts/3506624967.
They will be retained for up to 30 days.