Add sequencer-only benchmark role

[niran]

Summary

• add benchmark-level roles config with default sequencer+validator behavior
• support roles: [sequencer] to skip validator datadir setup, execution, log export, and metric requirements
• make validator metrics optional in run metadata and document the new snapshot/load-test use case

Testing

• go test ./...

[cb-heimdall]

🟡 Heimdall Review Status

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