Determine host, context, and core from update_endpoint on setConfiguration as well

I just realized this may not work due to how setBackendConfig works:

  #[ActionMethod(adminLabel: new TranslatableMarkup('Set backend config'), pluralize: FALSE)]
  public function setBackendConfig(array $backend_config) {
    $this->backend_config = $backend_config;
    // In case the backend plugin is already loaded, make sure the configuration
    // stays in sync.
    if ($this->backendPlugin
        && $this->getBackend()->getConfiguration() !== $backend_config) {
      $this->getBackend()->setConfiguration($backend_config);
    }
    return $this;
  }

So the parsing cannot be done unless the instance has been instantiated.