Document how to initialize a database with R2DBC

This commit adds a section to the reference guide on how to initialize
a database using R2DBC. 2 smoke tests are also added to validate this
behaviour with Flyway and Liquibase.

Closes gh-20742

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/r2dbc/R2dbcDatabaseInitializationExample.java

@@ -0,0 +1,50 @@
+/*
+ * 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.docs.r2dbc;
+
+import io.r2dbc.spi.ConnectionFactory;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.DefaultResourceLoader;
+import org.springframework.core.io.Resource;
+import org.springframework.core.io.ResourceLoader;
+import org.springframework.data.r2dbc.connectionfactory.init.ResourceDatabasePopulator;
+
+/**
+ * Example configuration for initializing a database using R2DBC.
+ *
+ * @author Stephane Nicoll
+ */
+public class R2dbcDatabaseInitializationExample {
+
+	// tag::configuration[]
+	@Configuration(proxyBeanMethods = false)
+	static class DatabaseInitializationConfiguration {
+
+		@Autowired
+		void initializeDatabase(ConnectionFactory connectionFactory) {
+			ResourceLoader resourceLoader = new DefaultResourceLoader();
+			Resource[] scripts = new Resource[] { resourceLoader.getResource("classpath:schema.sql"),
+					resourceLoader.getResource("classpath:data.sql") };
+			new ResourceDatabasePopulator(scripts).execute(connectionFactory).block();
+		}
+
+	}
+	// end::configuration[]
+
+}