Add WebClient.filter method

This commit introduces a new method on WebClient: filter, which takes an
filter function, and returns a (filtered) WebClient.

Issue: SPR-14961

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

@@ -95,6 +95,14 @@ class DefaultWebClientBuilder implements WebClient.Builder {
 							this.strategies));
 		}
 
+		@Override
+		public WebClient filter(ExchangeFilterFunction filter) {
+			Assert.notNull(filter, "'filter' must not be null");
+
+			ExchangeFilterFunction composedFilter = filter.andThen(this.filter);
+
+			return new DefaultWebClient(this.clientHttpConnector, this.strategies, composedFilter);
+		}
 	}
 
 	private class NoOpFilter implements ExchangeFilterFunction {

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

@@ -50,6 +50,15 @@ public interface WebClient extends ExchangeFunction {
 	@Override
 	Mono<ClientResponse> exchange(ClientRequest<?> request);
 
+	/**
+	 * Filters this client with the given {@code ExchangeFilterFunction}, resulting in a filtered
+	 * {@code WebClient}.
+	 * @param filterFunction the filter to apply to this client
+	 * @return the filtered client
+	 * @see ExchangeFilterFunction#apply(ExchangeFunction)
+	 */
+	WebClient filter(ExchangeFilterFunction filterFunction);
+
 
 	/**
 	 * Create a new instance of {@code WebClient} with the given connector. This method uses