Issue #2286357 by tim.plunkett: Introduce Display Variants, use for the block rendering flow.

core/config/schema/core.data_types.schema.yml

core/core.services.yml

core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php

core/lib/Drupal/Core/Display/VariantBase.php

core/lib/Drupal/Core/Display/VariantInterface.php

core/lib/Drupal/Core/Display/VariantManager.php

core/modules/block/block.module

...dules/block/src/Plugin/DisplayVariant/FullPageVariant.php

...k/tests/src/Plugin/DisplayVariant/FullPageVariantTest.php

core/modules/system/core.api.php

core/tests/Drupal/Tests/Core/Display/DisplayVariantTest.php