opt: add WITH operator

[jseldess]

PR: cockroachdb/cockroach#38670

From release notes:

Common Table Expressions (CTEs) may now be referenced from multiple locations in a query. [#38670][#38670] {% comment %}doc{% endcomment %}

This is also a roadmap item...
Airtable: https://airtable.com/tblD3oZPLJgGhCmch/viwchkdQkRUw6hZ3z/recpbDsGWkJvDakKU?blocks=hide
PR: cockroachdb/cockroach#21084