New plugin getter methods were added that do not throw exceptions

New variants of the plugin (and server instance) getter methods on our entity classes were added to improve DX for this common pattern:

    if ($index->hasValidTracker()) {
      return $index->getTrackerInstance()->…();
    }
    return [];

The following methods were added:

interface IndexInterface {
  public function getDatasourceIfAvailable(string $datasource_id): ?DatasourceInterface;
  public function getTrackerInstanceIfAvailable(): ?TrackerInterface;
  public function getServerInstanceIfAvailable(): ?ServerInterface;
  public function getProcessorIfAvailable(string $processor_id): ?ProcessorInterface;
}

interface ServerInterface {
  public function getBackendIfAvailable(): ?BackendInterface;
}

While the above example needs four lines and is (incorrectly) marked by IDEs as unsafe (because of an uncaught exception), this can now be written cleanly in the following way:

    return $index->getTrackerInstanceIfAvailable()?->…() ?? [];