Add auto-config for Spring Session cookie serializer

[vpavic]

This PR adds support for auto-configuring Spring Session's DefaultCookieSerializer using ServerProperties.

The auto-configuration of DefaultCookieSerializer will back-off is users provide their own CookieSerializer or HttpSessionStrategy bean.

This resolves #10371.

[wilkinsona]

or HttpSessionStrategy bean.

Is there a possibility that a user will want to use the auto-configured DefaultCookieSerializer to create a custom CookieHttpSessionStrategy?

[vpavic]

Thanks for the feedback @wilkinsona.

Is there a possibility that a user will want to use the auto-configured DefaultCookieSerializer to create a custom CookieHttpSessionStrategy?

Yes, that indeed is a possibility. We can only discard that option if user configured HttpSessionStrategy is an instance of HeaderHttpSessionStrategy.

[wilkinsona]

Thanks, @vpavic. Sounds like an update to the conditions may be in order. However, I think the preferred solution may be to fix #10699 although I'm not yet sure how to do so without adding something like ServletContextConfigurer that's subtly different to ServletContextInitializer.

[vpavic]

I agree that fixing #10699 would be the preferred solution.

Nevertheless, I'll update the discussed conditions to have this PR available as a backup plan.

[vpavic]

I'll update the discussed conditions to have this PR available as a backup plan.

Done.

[vpavic]

With #10699 resolved, this PR isn't needed any more.

I've tested the latest 2.0.0.BUILD-SNAPSHOT and auto-configuring Spring Session's DefaultCookieSerializer using ServerProperties now works as expected.

[wilkinsona]

Thanks for testing the snapshot, @vpavic. Much appreciated.