code: public ProxyBuilder<T> withInterface(Class<?> interfaceClass, Object interfaceImpl) {
      this.interfacesToImplementations.put(interfaceClass, interfaceImpl);
      return this;
    }
docs: Specifies an interfaces and an implementation of an interface by which the root object
  shall be extended.
  
  @param interfaceClass the class of the interface
  @param interfaceImpl  an implementation of the interface
  @return this builder for chaining.