PR #100949 - sql: add `unbounded_parallel_scans` session variable

Exalate commented:

Related PR: https://github.com/cockroachdb/cockroach/pull/100949
Commit: https://github.com/cockroachdb/cockroach/commit/9989de4c2a4d00fbf5943e986f19a65fc2208302
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-7432