opt: follow-up work for soft limit hints

[RaduBerinde]

List of remaining work items in the area of soft limits (see #34811):

• finalize and merge opt: factor limit hints into scan and lookup join costs #45604
• segmented sort
• better hints for streaming group-by, streaming distinct on
• follow up on review comments for opt: scale DISTINCT and SELECT limit hints based on statistics #42895
• pass hints through for set operations (variations on DISTINCT ON); UNION ALL should push to both sides
• joins (hash joins can push to left; merge joins)
• limit on limit (smaller higher-up limit): push smaller soft limit hint through hard limit
• Exists input should have soft-limit 1

Jira issue: CRDB-5279

[RaduBerinde]

Before further work here, it's worth revisiting our assumptions about how limits affect execution - these may be different for the new vectorized engine.