code: private <T extends Assignable> ExtensionWrapper loadExtensionMapping(Class<T> extensionClass) {
          final InputStream extensionStream = findExtensionImpl(extensionClass);
  
          ExtensionWrapper extensionWrapper = loadExtensionWrapper(extensionStream, extensionClass);
          this.extensionMappings.put(extensionClass, extensionWrapper);
          return extensionWrapper;
      }
docs: Finds the SPI configuration, wraps it into a {@link ExtensionWrapper} and loads it to
  {@link ServiceExtensionLoader#extensionMappings}.
  
  @param <T>
  @param extensionClass
  @return