Pause/resume capability for ThreadPoolTaskExecutor and ThreadPoolTaskScheduler

[jhoeller]

Motivated by checkpoint restore (#29921), ThreadPoolTaskExecutor and ThreadPoolTaskScheduler would benefit from a SmartLifecycle implementation which pauses and resumes task execution on stop and start, even if no SmartLifecycle-style auto-startup is needed here. This goes along with graceful shutdown enhancements in 6.1 (#27090, #24497) and can nicely be used on its own as well, either through direct calls on the executor bean or through ConfigurableApplicationContext.start/stop.

Task submitters may individually stop and restart their operations, so pause/resume behavior at the thread pool level is not strictly necessary for lifecycle alignment. However, independently submitted one-off tasks (@Async etc) and in particular periodic tasks (@Scheduled etc) can only really be centrally paused and resumed at the executor/scheduler level.