Add 'Global HTTP Client Configuration' reference docs section

Update documentation with information on how to configure the HTTP
client globally.

Closes gh-42888

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/io/restclient/clienthttprequestfactory/configuration/MyClientHttpConfiguration.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.io.restclient.clienthttprequestfactory.configuration;
+
+import java.net.ProxySelector;
+
+import org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration(proxyBeanMethods = false)
+public class MyClientHttpConfiguration {
+
+	@Bean
+	ClientHttpRequestFactoryBuilder<?> clientHttpRequestFactoryBuilder(ProxySelector proxySelector) {
+		return ClientHttpRequestFactoryBuilder.jdk()
+			.withHttpClientCustomizer((builder) -> builder.proxy(proxySelector));
+	}
+
+}

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/io/restclient/clienthttprequestfactory/configuration/MyClientHttpConfiguration.kt

@@ -0,0 +1,18 @@
+package org.springframework.boot.docs.io.restclient.clienthttprequestfactory.configuration
+
+import org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+import java.net.ProxySelector
+import java.net.http.HttpClient
+
+@Configuration(proxyBeanMethods = false)
+class MyClientHttpConfiguration {
+
+	@Bean
+	fun clientHttpRequestFactoryBuilder(proxySelector: ProxySelector): ClientHttpRequestFactoryBuilder<*>? {
+		return ClientHttpRequestFactoryBuilder.jdk()
+				.withHttpClientCustomizer { builder -> builder.proxy(proxySelector) }
+	}
+
+}