RFC: Storing data about URL when caching result

[silbinarywolf]

Planning to put together a PR but basically:

• Cache files will use the URL as the key.
• Variants of the page will be stored in this single file (json_encoded, key will be built using "updateCacheKeyFragments")
• File will also contain a "meta" key which allows a developer to store additional information about the page/URL when caching the result. (ie. the page type, controller type, page id, and potential flags, will most likely add an update hook. This information can be used during 'updateEnabled' or 'updateCacheKeyFragments' to change behaviour.

The use case for this is that I have a few page types which require more variants of the page to be cached. So I'd like to be able to only those variants to the cache key if it's applicable.

[silbinarywolf]

PR is too much of a big change and specific of a use case to go back in. And that's fine :)