Add static factory methods to WebClientAdapter

Ideally one would pass WebClient directly to HttpServiceProxyFactory,
but two need to remain decoupled. This commit adds static, shortcut
methods to WebClientAdapter to create an HttpServiceProxyFactory, thus
eliminating the step to wrap the WebClient.

src/docs/asciidoc/integration.adoc

@@ -390,14 +390,10 @@ the `WebClient`:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
-	WebClient client = WebClient.builder()
-			.baseUrl("https://api.github.com/")
-			.build();
+	WebClient client = WebClient.builder().baseUrl("https://api.github.com/").build();
+	HttpServiceProxyFactory factory = WebClientAdapter.createHttpServiceProxyFactory(client)).build();
 
-	HttpServiceProxyFactory proxyFactory = 
-			HttpServiceProxyFactory.builder(new WebClientAdapter(client)).build();
-
-	RepositoryService service = proxyFactory.createClient(RepositoryService.class);
+	RepositoryService service = factory.createClient(RepositoryService.class);
 ----
 
 An HTTP service interface can declare common attributes at the type level: