Add new config options to the Splunk storage plugin

  // Whether the Splunk client will validates the server's SSL cert.
  private final boolean validateCertificates;
  // The application context of the service.
  private final String app;
  // The owner context of the service.
  private final String owner;
  // A Splunk authentication token to use for the session.
  private final String token;
  // A valid login cookie.
  private final String cookie;