Introduce `ConfigurableApplicationContext.pause()` and `SmartLifecycle.isPauseable()`

[jhoeller]

Following up on #35171 and #35168, there are scenarios where a hard Lifecycle.stop() call on context pausing is undesirable, for example with ephemeral ports. This can currently be worked around through an early opt-out in the custom stop() implementation (see #35168 (comment)) but would benefit from a more obvious arrangement, also allowing for existing stop() behavior to remain as-is.

As a consequence, we're introducing a SmartLifecycle.isPauseable() method which defaults to true but can be switched to false based on certain configuration state in the component, or even always returning false for never participating in a pause attempt. The Test Context Framework calls ConfigurableApplicationContext.pause() now which takes isPauseable() into account, and ConfigurableApplicationContext.restart() also just stops pauseable components (if not paused yet) for its restart attempt.