PR #100948 - sql: add `unbounded_parallel_scans` session variable

[cockroach-teamcity]

Exalate commented:

Related PR: https://github.com/cockroachdb/cockroach/pull/100948 Commit: https://github.com/cockroachdb/cockroach/commit/ddba5f0ed197d34d56845e1c5c5d7d71db1c7835 Epic: CRDB-25448

---

Release note (sql change): New session variable unbounded_parallel_scans has been introduced which controls whether scans will be parallelized across ranges in more cases. Note that using this feature can lead to increased likelihood of OOMs, so it should be used with care (namely when you expect that the scan should read "reasonable" number of rows - probably less than 10k). Also note that queries with LIMITs aren't affected by this variable - cross-range parallelism of scans continues to be disabled for those.

Jira Issue: DOC-7479