Support benchmarking of Solr streaming expressions

[janhoy]

Summary

Solr Streaming Expressions
are a powerful distributed computation API, but Solr Orbit has no dedicated support for
benchmarking them. This issue tracks adding first-class streaming expression support.

Motivation

• Streaming expressions cover aggregation, faceting, parallel SQL, graph traversal, and
  machine learning scoring — workloads that differ significantly from standard query
  benchmarks
• Teams running Solr for analytics or ETL pipelines need a way to compare expression
  performance across Solr versions or cluster configurations

Proposed scope

1. New operation type: streaming-expression in workload definitions, analogous to
   the existing query and bulk-index operations
2. Dedicated driver: Execute expressions via /stream endpoint, collect timing and
   tuple-count metrics
3. Telemetry: Capture rows-returned, execution-time, and error rate per expression
4. Sample workload: Add an example workload in benchmarks/ that exercises common
   streaming patterns (search, facet, rollup, jdbc-like aggregation)

References

• https://solr.apache.org/guide/solr/latest/query-guide/streaming-expressions.html

[janhoy]

I'm not certain how much we need in this repo and how much is needed in a future streaming expr workload (operation type). But I believe we need some code here to know how to send streaming expr to solr...