As of spring-projects/spring-framework#24878, Spring Framework provides
an `ApplicationStartup` infrastructure that applications can use to
collect and track events during the application startup phase.

This commit adds a new `BufferingApplicationStartup` implementation that
buffer `StartupStep`s and tracks their execution time. Once buffered,
these steps can be pushed to an external metrics system or drained
through a web endpoint, to a file...

Closes gh-22603