SpringBoot Provide a sessions endpoint for WebFlux applications

Spring Session 3.3 introduces a new ReactiveFindByIndexNameSessionRepository API that we can use to provide a sessions endpoint for WebFlux applications.

Comment From: vpavic

Reminder that https://github.com/spring-projects/spring-boot/pull/32046 does that (in basic form) even without the new reactive indexed repos. That PR should probably be updated to leverage the new APIs though.

Comment From: mhalbritter

Superseded by #32046.