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-starters/pom.xml

@@ -38,6 +38,7 @@
 		<module>spring-boot-starter-data-mongodb-reactive</module>
 		<module>spring-boot-starter-data-neo4j</module>
 		<module>spring-boot-starter-data-redis</module>
+		<module>spring-boot-starter-data-redis-reactive</module>
 		<module>spring-boot-starter-data-rest</module>
 		<module>spring-boot-starter-data-solr</module>
 		<module>spring-boot-starter-freemarker</module>