Include better support for cache purging on page caching layer

Use case:
We want to implement longer ttl caches + cache purging for some selected API request paths (generally single post/page/cpt api responses)

In order to support this, we need to be able to purge our `object caches`, the `page cache` and the `CDN cache`

Currently the page caching layer has 2 main limitations which make this difficult.

- Batache does not support s-maxage, Joe has made a PR for this already https://github.com/humanmade/batcache/pull/26 . `s-maxage` control is a requirement in order to allow long TTL CDN caching with short TTL browser caching
- Currently batcache.php is not loaded via altis cloud, only advanced-cache.php
This means the utility functions are not easily accessible, and have to be manually loaded from vendor, or code copied as custom functionality if we want to manually purge a page path from batcache --- this is a requirement to enable CDN cache purging with page caching enabled, as batcache url also needs to be purged beforehand to avoid CDN caching an old version of the page