code: public static final Builder privateKey(Class<?> cls, String resource) {
          return new Builder().privateKey(cls, resource);
      }
docs: Returns a builder having loaded the private key from the classpath
  relative to the classloader used by {@code cls}.
  
  @param cls
  the class whose classloader is used to load the resource
  @param resource
  the resource path
  @return the PPK builder