Issue #2256355 by tim.plunkett: Make block plugins usable outside the block entity context.

core/modules/block/block.services.yml

...lib/Drupal/custom_block/Plugin/Block/CustomBlockBlock.php

core/modules/block/lib/Drupal/block/BlockBase.php

core/modules/block/lib/Drupal/block/BlockListBuilder.php

core/modules/block/lib/Drupal/block/BlockManager.php

.../modules/block/lib/Drupal/block/BlockManagerInterface.php

core/modules/block/lib/Drupal/block/BlockPluginInterface.php

...rupal/block/Controller/CategoryAutocompleteController.php

...dules/block/lib/Drupal/block/Tests/BlockInterfaceTest.php

...les/block/lib/Drupal/block/Tests/BlockStorageUnitTest.php

...ock/tests/Drupal/block/Tests/CategoryAutocompleteTest.php

...age/lib/Drupal/language/Form/NegotiationConfigureForm.php