Issue #2016589 by amateescu, tim.plunkett: Add a dedicated @EntityReferenceSelection annotation.

...nt/Plugin/entity_reference/selection/CommentSelection.php

core/modules/entity_reference/entity_reference.module

core/modules/entity_reference/entity_reference.services.yml

.../entity_reference/Annotation/EntityReferenceSelection.php

...l/entity_reference/Plugin/Type/SelectionPluginManager.php

...rence/Plugin/entity_reference/selection/SelectionBase.php

.../file/Plugin/entity_reference/selection/FileSelection.php

.../node/Plugin/entity_reference/selection/NodeSelection.php

...onomy/Plugin/entity_reference/selection/TermSelection.php

.../user/Plugin/entity_reference/selection/UserSelection.php

...iews/Plugin/entity_reference/selection/ViewsSelection.php