Add JFR integration to detect virtual thread pinning events and report upon them.

https://mikemybytes.com/2024/04/17/continuous-monitoring-of-pinned-threads-with-spring-boot-and-jfr/ provides some information.

A possible place to do this is in the core-management subsystem, via a new resource that turns on the feature and can be used to configure the reporting. Initial report could be via logging at a configurable level.

Project Loom may make most pinning issues go away[1], so we should resist the temptation to go crazy with this. I think the key thing is if we let users use virtual threads in any context where pinning is a concern (as it is in any current main-line JDK), then we need to provide a means for them to be aware of pinning. It's clear libraries will have pinning issues, so users would need to know if their potential virtual-thread workloads will experience them.

[1] https://mail.openjdk.org/pipermail/loom-dev/2024-February/006433.html