Optimization: do not iterate on $entity fields in EntityViewBuilder::viewField()

Coming from #2313875: Preserve the 'field_type' within FieldInstanceConfig, which tracks the places where we load FieldStorageDefinition objects while in the "ideal" critical path of "entity render with entity cache and render cache hits".

On node/[nid], EntityViewController::view() (page controller for "full entity" pages) does an explicit $entity->title->view('full'), to extract the entity title as page title.

This currently triggers full instantiation of the FieldItemList objects (which in turn requires loading the FieldStorageDefinition objects) for *all* the fields in the entity, just because EntityViewBuilder() does a foreach($entity) to iterate on the fields present in the entity.

This could be avoided alltogether if we iterated on $entity->getFieldDefinitions() instead.