Cache context support

Problem/Motivation

Decoupled Toolbox does not yet support cache contexts, so we don't get variants per context when having the cache enabled.

Workaround

Disable the cache preemptively (before rendering) with disableCache() when cache contexts are expected.

Proposed resolution

Enhance the cache and render system.

Each rendered entity is cached. We have to take care about duplicates. To limit such duplicates, a placeholder system is required.

This placeholder system will prevent the renderer to go further when a cache context is used by a FieldFormatter, and mark the location with something analogous to the Lazy Builder used by dynamic page cache.

Warning! Make sure that this does interact well with the "field location" system that can relocate data for each field after the pre-render phase.

Remaining tasks

All!

API changes

We have only tags supported right now, so every method relying on tags have to be augmented with contexts support, usually by converting tags array by a CacheableMetadata object that will hold all cache related info.

Data model changes

Cached data is expected to get their cache ID (key) changed. The new VariationCache system will be required as an intermediate.

So a cache rebuild will be required upon update to get rid of stale data.