Issue #2443615 by benjy: Add an interface to MigrateExecutable

core/modules/migrate/src/MigrateExecutable.php

core/modules/migrate/src/MigrateExecutableInterface.php

core/modules/migrate/src/Plugin/MigrateProcessInterface.php

core/modules/migrate/src/Plugin/migrate/process/Callback.php

core/modules/migrate/src/Plugin/migrate/process/Concat.php

...modules/migrate/src/Plugin/migrate/process/DedupeBase.php

...dules/migrate/src/Plugin/migrate/process/DefaultValue.php

core/modules/migrate/src/Plugin/migrate/process/Extract.php

core/modules/migrate/src/Plugin/migrate/process/Flatten.php

core/modules/migrate/src/Plugin/migrate/process/Get.php

core/modules/migrate/src/Plugin/migrate/process/Iterator.php

...odules/migrate/src/Plugin/migrate/process/MachineName.php

.../modules/migrate/src/Plugin/migrate/process/Migration.php

core/modules/migrate/src/Plugin/migrate/process/Route.php

...migrate/src/Plugin/migrate/process/SkipProcessOnEmpty.php

...es/migrate/src/Plugin/migrate/process/SkipRowIfNotSet.php

...les/migrate/src/Plugin/migrate/process/SkipRowOnEmpty.php

.../modules/migrate/src/Plugin/migrate/process/StaticMap.php

core/modules/migrate/src/Source.php

...te_drupal/src/Plugin/migrate/process/d6/BlockPluginId.php