Add customizer variant of requestFactorySettings

Add `requestFactorySettings` that accepts a `UnaryOperator` so that
the existing `requestFactorySettings` can be customized rather than
replaced.

Closes gh-43258

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplate.java

@@ -25,6 +25,7 @@ import java.time.Duration;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
+import java.util.function.UnaryOperator;
 
 import javax.net.ssl.SSLContext;
 
@@ -974,6 +975,22 @@ public class TestRestTemplate {
 				this.restTemplate.getUriTemplateHandler());
 	}
 
+	/**
+	 * Creates a new {@code TestRestTemplate} with the same configuration as this one,
+	 * except that it will customize the {@link ClientHttpRequestFactorySettings}. The
+	 * request factory used is a new instance of the underlying {@link RestTemplate}'s
+	 * request factory type (when possible).
+	 * @param requestFactorySettingsCustomizer a {@link UnaryOperator} to update the
+	 * settings
+	 * @return the new template
+	 * @since 3.4.1
+	 */
+	public TestRestTemplate withRequestFactorySettings(
+			UnaryOperator<ClientHttpRequestFactorySettings> requestFactorySettingsCustomizer) {
+		return new TestRestTemplate(this.builder.requestFactorySettings(requestFactorySettingsCustomizer),
+				this.restTemplate.getUriTemplateHandler());
+	}
+
 	@SuppressWarnings({ "rawtypes", "unchecked" })
 	private RequestEntity<?> createRequestEntityWithRootAppliedUri(RequestEntity<?> requestEntity) {
 		return new RequestEntity(requestEntity.getBody(), requestEntity.getHeaders(), requestEntity.getMethod(),