fix: collect StarRocks explain analyze plans

[zgxme]

Summary

• collect one StarRocks EXPLAIN ANALYZE output for each query after its timed runs finish
• write analyze plan text to plan/_analyze_plan.txt
• add an optional engine_get_analyze_plan hook to the engine interface

Verification

• bash -n benchmark.sh
• bash -n engines/starrocks_engine.sh
• bash -n engines/interface.sh
• git diff --check -- benchmark.sh engines/starrocks_engine.sh engines/interface.sh
• fake mysql smoke verified query1 runs, then query1 EXPLAIN ANALYZE, then query2 runs, then query2 EXPLAIN ANALYZE