Issue #1967294 by tim.plunkett: Add a dedicated @EntityType annotation.

core/lib/Drupal/Component/Annotation/Plugin.php

core/lib/Drupal/Core/Entity/Annotation/EntityType.php

core/lib/Drupal/Core/Entity/EntityManager.php

...regator/lib/Drupal/aggregator/Plugin/Core/Entity/Feed.php

...regator/lib/Drupal/aggregator/Plugin/Core/Entity/Item.php

...ib/Drupal/custom_block/Plugin/Core/Entity/CustomBlock.php

...rupal/custom_block/Plugin/Core/Entity/CustomBlockType.php

...dules/block/lib/Drupal/block/Plugin/Core/Entity/Block.php

...t/lib/Drupal/breakpoint/Plugin/Core/Entity/Breakpoint.php

.../Drupal/breakpoint/Plugin/Core/Entity/BreakpointGroup.php

...comment/lib/Drupal/comment/Plugin/Core/Entity/Comment.php

...Drupal/config_test/Plugin/Core/Entity/ConfigQueryTest.php

.../lib/Drupal/config_test/Plugin/Core/Entity/ConfigTest.php

...onfig_test/Plugin/Core/Entity/ConfigTestEmptyManifest.php

...ontact/lib/Drupal/contact/Plugin/Core/Entity/Category.php

...contact/lib/Drupal/contact/Plugin/Core/Entity/Message.php

...es/editor/lib/Drupal/editor/Plugin/Core/Entity/Editor.php

...ty/lib/Drupal/entity/Plugin/Core/Entity/EntityDisplay.php

...dules/field/lib/Drupal/field/Plugin/Core/Entity/Field.php

...eld/lib/Drupal/field/Plugin/Core/Entity/FieldInstance.php