Add auto-configuration for reactive Redis

This commit provides an auto-configuration for reactive Redis and a
starter that provides Lettuce as Jedis doesn't support reactive
operations.

There are no support for reactive redis repositories at the moment so
only a `ReactiveRedisTemplate` is auto-configured if necessary.

Closes gh-8053

spring-boot-dependencies/pom.xml

@@ -389,6 +389,11 @@
 				<artifactId>spring-boot-starter-data-redis</artifactId>
 				<version>2.0.0.BUILD-SNAPSHOT</version>
 			</dependency>
+			<dependency>
+				<groupId>org.springframework.boot</groupId>
+				<artifactId>spring-boot-starter-data-redis-reactive</artifactId>
+				<version>2.0.0.BUILD-SNAPSHOT</version>
+			</dependency>
 			<dependency>
 				<groupId>org.springframework.boot</groupId>
 				<artifactId>spring-boot-starter-data-neo4j</artifactId>