[SITE-2506] Add filter to adjust cache max age for specific paths

[pwtyler]

Use case: adjust the cache max-age for REST API endpoints, but not the main website, without adjusting the global default.

Potential approach using rest_post_dispatch today is described in the ChatGPT conversation linked in this support topic, this could be simplified with a filter that fires after the default value is set.

[pwtyler]

Discovered the rest_post_dispatch option is already documented on docs.pantheon.io

https://docs.pantheon.io/cache-control