Restore RestClientBuilderCustomizer in its original location

This commit restores RestClientBuilderCustomizer in the rest package in
a deprecated fashion so that the upgrade from 2.2 is smoother.

Closes gh-21572

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/rest/RestClientBuilderCustomizer.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2012-2020 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.autoconfigure.elasticsearch.rest;
+
+import org.elasticsearch.client.RestClientBuilder;
+
+/**
+ * Callback interface that can be implemented by beans wishing to further customize the
+ * {@link org.elasticsearch.client.RestClient} via a {@link RestClientBuilder} whilst
+ * retaining default auto-configuration.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ * @deprecated as of 2.3.1 in favor of
+ * {@link org.springframework.boot.autoconfigure.elasticsearch.RestClientBuilderCustomizer}
+ */
+@FunctionalInterface
+@Deprecated
+public interface RestClientBuilderCustomizer
+		extends org.springframework.boot.autoconfigure.elasticsearch.RestClientBuilderCustomizer {
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/elasticsearch/ElasticsearchRestClientAutoConfigurationTests.java

@@ -113,6 +113,16 @@ void configureWhenBuilderCustomizerShouldApply() {
 		});
 	}
 
+	@Test
+	@Deprecated
+	void configureWhenDeprecatedBuilderCustomizerShouldApply() {
+		this.contextRunner.withUserConfiguration(DeprecatedBuilderCustomizerConfiguration.class).run((context) -> {
+			assertThat(context).hasSingleBean(RestClient.class);
+			RestClient restClient = context.getBean(RestClient.class);
+			assertThat(restClient).hasFieldOrPropertyWithValue("pathPrefix", "/deprecated");
+		});
+	}
+
 	@Test
 	void configureWithNoTimeoutsApplyDefaults() {
 		this.contextRunner.run((context) -> {
@@ -193,6 +203,17 @@ public void customize(RequestConfig.Builder builder) {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@Deprecated
+	static class DeprecatedBuilderCustomizerConfiguration {
+
+		@Bean
+		org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientBuilderCustomizer myCustomizer() {
+			return (builder) -> builder.setPathPrefix("/deprecated");
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class CustomRestHighLevelClientConfiguration {