Add cloneBuilder method on WebClient.Builder

This commit adds a new `cloneBuilder()` method on `WebClient.Builder`;
we can now reuse the customizations of an existing builder without
sharing its state across several `WebClient` building code paths.

Issue: SPR-15690

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClientBuilder.java

@@ -92,6 +92,11 @@ class DefaultWebClientBuilder implements WebClient.Builder {
 		return this;
 	}
 
+	@Override
+	public WebClient.Builder cloneBuilder() {
+		return new DefaultWebClientBuilder(this);
+	}
+
 	@Override
 	public WebClient.Builder defaultUriVariables(Map<String, ?> defaultUriVariables) {
 		this.defaultUriVariables = defaultUriVariables;

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/WebClient.java

@@ -192,6 +192,11 @@ public interface WebClient {
 		 */
 		Builder baseUrl(String baseUrl);
 
+		/**
+		 * Clone this {@code WebClient.Builder}
+		 */
+		Builder cloneBuilder();
+
 		/**
 		 * Configure default URI variable values that will be used when expanding
 		 * URI templates using a {@link Map}.