[CMSP-644] add a Option field for REST API TTL

[pwtyler]

Copying over pantheon-systems/WordPress#198 to evaluate since that was initially proposed before the mu plugin was broken out from the upstream.

Fom @eabquina—

For a general use case where a customer wants to set a specific cache TTL for WP REST API endpoints, we add a specific field for the REST API TTL value

Alternatively, we can implement a custom plugin to set specific cache headers for these endpoints too, but that will be covered by another proposal or docs guide

The intention here is to avoid modifying wp-config.php to achieve this, and specifically for those who want to disable cache for

See pantheon-systems/WordPress@57ba357 for the specific diff that should be re-applied if we pursue this.