Issue #1969692 by dawehner: Add a dedicated annotation for aggregator Plugins.

...or/lib/Drupal/aggregator/Annotation/AggregatorFetcher.php

...tor/lib/Drupal/aggregator/Annotation/AggregatorParser.php

.../lib/Drupal/aggregator/Annotation/AggregatorProcessor.php

.../lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php

...l/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php

...pal/aggregator/Plugin/aggregator/parser/DefaultParser.php

...gregator/Plugin/aggregator/processor/DefaultProcessor.php

...aggregator_test/Plugin/aggregator/fetcher/TestFetcher.php

...l/aggregator_test/Plugin/aggregator/parser/TestParser.php

...egator_test/Plugin/aggregator/processor/TestProcessor.php