code: public Object setExtensionProperty(String key, Object value)
      {
          return getExtensionProperties().put(key, value);
      }
docs: Sets a custom extension property to be set on each of the extensions that are going to be installed from this
  request.
  
  @param key the property name
  @param value the new property value
  @return the previous property value
  @since 7.0M2