In some cases we want to load a configuration file from optional / install folders. For example to compare it with active config and apply new changes.
To load a configuration from install folder user the next code snippet.

use Drupal\Core\Config\ExtensionInstallStorage;

$field_storage = 'field.storage.group_content.grequest_status';
$install_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_INSTALL_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);

From optional folder

use Drupal\Core\Config\ExtensionInstallStorage;

$name = 'views.view.group_pending_members';

$optional_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_OPTIONAL_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);

and schema configuration

use Drupal\Core\Config\ExtensionInstallStorage;

$name = 'views.view.group_pending_members';

$optional_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_SCHEMA_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);