New interface added to ensure request context-dependent values do not break cacheability

The following interface should be implemented by any class whose business logic implies dealing at runtime with variations of a canonical object:

interface RefinableCacheableDependencyInterface {

  public function addCacheContexts(array $cache_contexts);
  public function addCacheTags(array $cache_tags);
  public function mergeCacheMaxAge($max_age);


}

Typical examples are:

• Code relying on the current content language to decided which entity translation should be displayed. EntityManager::getTranslationFromContext() now automatically takes care of these cases, that is when no language parameter is explicitly provided.
• Configuration overrides: in core configuration may vary by interface language, which is a required cache context. However contrib can implement additional overrides, for instance Domain Access or Organic Groups: these will require to specify their additional cache contexts dependencies at runtime.