Add support for providing a custom SessionTimeout bean

[bartsopers]

This change allows for customization of Spring Session session timeout in Spring Boot Session.

Closes #49882

[snicoll]

@bartsopers SessionTimeout is a by-product of the modularization and isn't really meant to be overridden by the user (which explains why it doesn't have a condition to override it).

Looking at the implementations, why can't you set the relevant property in the Environment? We'd need more details before discussing the merit of the PR.

[bartsopers]

@snicoll our use case is that our on-premise customers can configure session management (either Spring Session with redis or servlet container backed sessions) through spring profiles. This would mean that, depending on their configuration and their deployment type (we offer both jar and war artifacts), they would either need to use the server.servlet.session.timeout or the spring.session.timeout property. This is confusing for our customers, so we want to introduce a single property to set the session timeout.
In the Spring Session case, imho, providing a Spring bean would feel more robust than programmatically setting properties in the Environment based on other properties.

[snicoll]

@bartsopers fair enough, we can consider this an added value of the modularization.