Restructure Redis HttpSession configuration support

[vpavic]

The purpose of this issue is to revisit the Redis HttpSession configuration arrangement put in place in #1711. The @EnableRedisHttpSession attribute enableIndexingAndEvents introduced in that issue is a binary choice and as such is very fragile because it won't tolerate any additional SessionRepository implementations.

Goals:

• provide configuration support for the new default (RedisSessionRepository) while allowing to easily opt into the previous default (RedisIndexedSessionRepository)
• aim to provide extensible configuration support that will be friendly to additional SessionRepository implementations
• consider Spring Boot's auto-configuration support and ensure it's also able to opt into the previous defaults using configuration properties

Related issues:

• Adapt to Spring Session Data Redis switching to an unindexed repository by default spring-boot#30673