Speed up application startup times with AppCDS

[groldan]

Enable AppCDS creation during Docker image build.

Improves application startup times from ~10% to ~30% on a fast machine. The improvement is expected to be more noticeable on slower hosts. That said, a big part of a GeoServer application startup time relates to catalog and configuration initialization, and in the case of GeoServer Cloud, there's also the Spring Cloud initialization (e.g. rabbitmq, config service lookup and discovery service registration). Nonetheless, AppCDS greatly alleviates the time spent loading and compiling classes, which relieves the host from having several containers starting up and competing from resources just for that.

AppCDS (Application Class Data Sharing) is a JVM feature that allows preloading and sharing class metadata across JVM instances. This can significantly improve startup time and reduce memory usage, especially for large applications with many dependencies.

This patch launches the applications during the Docker image build process for a short period of time (until the spring context is first refreshed) to build the AppCDS archive, and introduces an AutoConfiguration that:

• Allows passing a JVM argument (-Dspring.context.exit=<event>) to terminate the application upon specific Spring ApplicationContextEvent events.
• Facilitates starting the application during Docker image builds to create the AppCDS archive, improving startup performance.

Supported events include:

• onPrepared: The SpringApplication is starting up, and the ApplicationContext is fully prepared but not yet refreshed. The bean definitions are loaded, and the Environment is ready for use at this stage.
• onRefreshed: ApplicationContext gets initialized or refreshed.
• onStarted: ApplicationContext has been refreshed, but before any ApplicationRunner or CommandLineRunner instances have been called.
• onReady: Published as late as possible to indicate that the application is ready to service requests. All initialization steps will have been completed.

Note: Spring Boot 3.2+ natively supports spring.context.exit=onRefresh as of this commit, so this feature may not be necessary post-upgrade from Spring Boot 2.7.

Additional events are included for cases where certain applications may require different startup stages for proper initialization. The new offline embedded Spring profile should also facilitate starting without Spring Cloud Bus, ACL, etc.