bigquery: read large unordered ARROW results with more than one stream #18804

Before ARROW, to_arrow() and to_dataframe() read query results with as many streams as BigQuery allows unless _contains_order_by() matched the query (_download_table_bqstorage() requests 0 streams, which means unlimited). ARROW query results always read with one stream, so very large unordered results may be slower than on the old path. Raised by @tswast in #18654 (comment).

• Explicit destination tables (feat(bigquery): read Arrow results from explicit destinations #18654): CreateReadSession uses max_stream_count=1. Do the same _contains_order_by() check and allow more streams when the query has no ORDER BY, reading them in parallel.
• Job _default stream (jobs.query, or jobs.insert with an anonymous destination): read directly with no read session, so there's no stream count. Check whether SplitReadStream works on job default streams, or whether there's another way to parallelize.
• Compare throughput against the non-ARROW path for large unordered results.

Still confirming whether very large results are in scope for the ARROW work. The Node.js client (createQueryArrowStream()) has the same design.