PR #91228 - sql: add partial statistics USING EXTREMES to system.table_statistics

[cockroach-teamcity]

Related PR: https://github.com/cockroachdb/cockroach/pull/91228 Commit: https://github.com/cockroachdb/cockroach/commit/0c2c58376141a297d1e5192bd5a2941312747bb8

---

Release note (sql change): users can now manually create partial single-column statistics at the extreme values on columns that are prefixes of their index. The output of SHOW STATISTICS now includes a column indicating the partial predicate for a partial statistic, or NULL for a full statistic.