code: public RestTemplateBuilder requestFactory(
  			Supplier<ClientHttpRequestFactory> requestFactorySupplier) {
  		Assert.notNull(requestFactorySupplier,
  				"RequestFactory Supplier must not be null");
  		return new RestTemplateBuilder(this.detectRequestFactory, this.rootUri,
  				this.messageConverters, requestFactorySupplier, this.uriTemplateHandler,
  				this.errorHandler, this.basicAuthentication, this.restTemplateCustomizers,
  				this.requestFactoryCustomizer, this.interceptors);
  	}
docs: Set the {@code Supplier} of {@link ClientHttpRequestFactory} that should be called
  each time we {@link #build()} a new {@link RestTemplate} instance.
  @param requestFactorySupplier the supplier for the request factory
  @return a new builder instance
  @since 2.0.0