Replace WebClient.filter with Builder.filter

This commit replaces the WebClient.filter method with
WebClient.Builder.filter. The reason for this change is that filters
added via WebClient.filter would be applied in the opposite order of
their declaration, due to the compositional nature of the method,
combined with the immutable nature of the WebClient.
WebClient.Builder.filter does keep the order of the filters, as
registered.

Furthermore, this commit introduces a WebClient.mutate() method,
returning a WebClient.Builder. This method allow to add/remove filters
and other defaults from a given WebClient.

Issue: SPR-15657

Add WebClient.Builder.addFilter

Add Consumer-based headers and cookies methods to builders.

Add WebClient.mutate

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/ExchangeMutatorWebFilterTests.java

@@ -59,8 +59,14 @@ public class ExchangeMutatorWebFilterTests {
 
 	@Test
 	public void perRequestMutators() throws Exception {
-		this.webTestClient
+
+		this.webTestClient = WebTestClient.bindToController(new TestController())
+				.webFilter(this.exchangeMutator)
+				.configureClient()
 				.filter(this.exchangeMutator.perClient(userIdentity("Giovanni")))
+				.build();
+
+		this.webTestClient
 				.get().uri("/userIdentity")
 				.exchange()
 				.expectStatus().isOk()