Add BE stack trace monitor

[aytrack]

Summary

• Start a background BE stack trace monitor during benchmark workload execution.
• Collect GET /api/stack_trace from each BE using be_http_port every 40 seconds by default without failing the workload.
• Save per-BE trace files under the benchmark result directory, log every saved trace path, archive the traces, and upload the archive to the configured file server endpoint.

Trace output

• Local files: $RESULT_DIR/stack_trace/<be_host>/<YYYYmmdd_HHMMSS>.txt
• Archive: $RESULT_DIR/stack_trace_$TIMESTAMP.tar.gz
• Uploaded URL is printed in execution logs as Stack trace archive URL: ...

Validation

• bash tests/test_stack_trace_monitor.sh
• bash -n benchmark.sh && bash -n tests/test_stack_trace_monitor.sh
• git diff --check