Make JFR-based observability opt-in

[fabiomolignoni]

Fixes #5326.

Spring Batch currently instantiates JFR events directly, causing NoClassDefFoundError when running on minimized Java runtimes without the jdk.jfr module.

This change introduces a JFR-independent BatchEventRecorder abstraction with a no-op default implementation. FlightRecorderBatchEventRecorder provides explicit JFR activation, with recorder configuration supported through the application context, builders, and setters.
This follows the same opt-in approach as Spring Framework's ApplicationStartup infrastructure, which uses a no-op implementation by default and provides FlightRecorderApplicationStartup for explicitly enabling JFR startup events.
Existing batch event instrumentation delegates to the new abstraction, and the documentation explains how to enable JFR recording.

Validation:

• Added unit tests for the no-op recorder, JFR event factories, recorder injection, and job launch recording.
• Manually verified that tasklet and chunk jobs complete with jdk.jfr unavailable.
• Manually verified that explicit JFR activation produces recorded batch events.