Rework configuration classes

[vpavic]

Spring Session's configuration infrastructure relies a lot on inheritance, with HazelcastHttpSessionConfiguration, JdbcHttpSessionConfiguration and RedisHttpSessionConfiguration all extending SpringHttpSessionConfiguration, and RedisWebSessionConfiguration extending SpringWebSessionConfiguration. We should rework these so that they rely on @Import instead, which should ensure they are easier to evolve in the long term.

This will also impact Spring Boot as its auto-configuration support for Spring Session is based on configuration class inheritance.