Add Lettuce Redis driver autoconfiguration

Introduce an alternative autoconfiguration if the lettuce Redis driver is
available. Add Lettuce-specific configuration property options
"spring.redis.lettuce.shutdown-timeout" to control the shutdown timeout
of the lettuce driver. Add documentation for the properties, the
supported drivers, and how to switch between drivers.

Split client-specific properties from spring.redis.pool to
spring.redis.jedis.pool and introduce spring.redis.lettuce namespace.
Deprecate spring.redis.pool property.

See gh-5311

spring-boot-docs/pom.xml

@@ -748,6 +748,11 @@
 			<artifactId>jedis</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>io.lettuce</groupId>
+			<artifactId>lettuce-core</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<!-- Test dependencies -->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>