Restructure Redis HttpSession configuration support

This commit restructures configuration support for Redis-backed
HttpSession with aim to enable users to easily select the
SessionRepository implementation they prefer to use.

This is achieved by introducing [at]EnableRedisIndexedHttpSession
annotation that can be used to configure RedisIndexedSessionRepository,
while the existing [at]EnableRedisHttpSession will going forward
configure RedisSessionRepository as the SessionRepository implementation
used by Spring Session.

Additionally, this also introduces AbstractRedisHttpSessionConfiguration
as the base configuration class that manages common aspects of
Redis-backed HttpSession support, which is then extended by more
specific configuration classes that provide specific SessionRepository
implementation.

Closes gh-2122

spring-session-docs/modules/ROOT/examples/java/docs/IndexDocTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 the original author or authors.
+ * Copyright 2014-2022 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.
@@ -130,7 +130,7 @@ class IndexDocTests {
 	@SuppressWarnings("unused")
 	void newRedisIndexedSessionRepository() {
 		// tag::new-redisindexedsessionrepository[]
-		RedisTemplate<Object, Object> redisTemplate = new RedisTemplate<>();
+		RedisTemplate<String, Object> redisTemplate = new RedisTemplate<>();
 
 		// ... configure redisTemplate ...