sql, config: make zone configs cascade

[jseldess]

PR: cockroachdb/cockroach#30611

From release notes:

Updated the output of SHOW ZONE CONFIGURATIONS. Also, unset fields in zone configurations now inherit parent values. [#30611][#30611] {% comment %}doc{% endcomment %}

[ridwanmsharif]

I notice there being a lot of warnings all over our Docs about how changes made to a zone config,
doesn't "cascade" down. The changes don't retroactively apply to all zone configs that inherited from it. With the changes made in cockroachdb/cockroach#30611, they now do exactly that. For more detail, the RFC talks about the problem and our solution: https://github.com/cockroachdb/cockroach/blob/master/docs/RFCS/20180919_cascading_zone_configs.md