Support EXPLAIN ANALYZE with creating unspecified partial stats

We require `reqStats` to have only 1 element in `createPartialStatsPlan()`
https://github.com/cockroachdb/cockroach/blob/c9f233b4778cfaee864e8e44518fd33a7f5476b0/pkg/sql/distsql_plan_stats.go#L281

This works for `CREATE STATISTICS FROM USING EXTREMES` where we don't specify a column since we plan and run create stats separately for each column in the default set. However, this doesn't support `EXPLAIN ANALYZE CREATE STATISTICS FROM USING EXTREMES` and certain other EXPLAINS (such as `EXPLAIN (VEC)`) since we pass job details with all of the columns in the default column set together and expect a single plan: https://github.com/cockroachdb/cockroach/blob/34057369826b8b9384b4f20a38cbce55bded5d81/pkg/sql/distsql_physical_planner.go#L3802

Jira issue: CRDB-41268