Issue #2028037 by marvin_B8, LinL, jsbalsera, ceng, Berdir: Expand FeedInterface with methods.

core/modules/aggregator/aggregator.module

core/modules/aggregator/aggregator.pages.inc

.../modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php

...s/aggregator/lib/Drupal/aggregator/FeedFormController.php

...odules/aggregator/lib/Drupal/aggregator/FeedInterface.php

...les/aggregator/lib/Drupal/aggregator/Form/OpmlFeedAdd.php

...regator/lib/Drupal/aggregator/Plugin/FetcherInterface.php

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

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

...es/aggregator/lib/Drupal/aggregator/Tests/AddFeedTest.php

...egator/lib/Drupal/aggregator/Tests/AggregatorTestBase.php

...tor/lib/Drupal/aggregator/Tests/FeedFetcherPluginTest.php

...aggregator/lib/Drupal/aggregator/Tests/FeedParserTest.php

...r/lib/Drupal/aggregator/Tests/FeedProcessorPluginTest.php

...aggregator/lib/Drupal/aggregator/Tests/RemoveFeedTest.php