Integrate Apache http client with WebClient

This commit introduces a ClientHttpConnector implementation backed by
Apache HttpComponents HttpClient 5.0.

Fixes gh-24700

src/docs/asciidoc/web/webflux-webclient.adoc

@@ -369,6 +369,33 @@ shows:
 <2> Plug the connector into the `WebClient.Builder`.
 
 
+
+[[webflux-client-builder-http-components]]
+=== HttpComponents
+
+The following example shows how to customize Apache HttpComponents `HttpClient` settings:
+
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+.Java
+----
+	HttpAsyncClientBuilder clientBuilder = HttpAsyncClients.custom();
+	clientBuilder.setDefaultRequestConfig(...);
+	CloseableHttpAsyncClient client = clientBuilder.build();
+	ClientHttpConnector connector = new HttpComponentsClientHttpConnector(client);
+
+	WebClient webClient = WebClient.builder().clientConnector(connector).build();
+----
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+.Kotlin
+----
+	val client = HttpAsyncClients.custom().apply {
+		setDefaultRequestConfig(...)
+	}.build()
+	val connector = HttpComponentsClientHttpConnector(client)
+	val webClient = WebClient.builder().clientConnector(connector).build()
+----
+
+
 [[webflux-client-retrieve]]
 == `retrieve()`
 

src/docs/asciidoc/web/webflux.adoc

@@ -312,8 +312,9 @@ request handling, on top of which concrete programming models such as annotated
 controllers and functional endpoints are built.
 * For the client side, there is a basic `ClientHttpConnector` contract to perform HTTP
 requests with non-blocking I/O and Reactive Streams back pressure, along with adapters for
-https://github.com/reactor/reactor-netty[Reactor Netty] and for the reactive
-https://github.com/jetty-project/jetty-reactive-httpclient[Jetty HttpClient].
+https://github.com/reactor/reactor-netty[Reactor Netty], reactive
+https://github.com/jetty-project/jetty-reactive-httpclient[Jetty HttpClient]
+and https://hc.apache.org/[Apache HttpComponents].
 The higher level <<web-reactive.adoc#webflux-client, WebClient>> used in applications
 builds on this basic contract.
 * For client and server, <<webflux-codecs, codecs>> for serialization and