Allow reactive caching to be used in services

What does this MR do?

The ReactiveCachingWorker currently relies on the ActiveRecord#find_by method to get the original object. This limits use of ReactiveCaching to only ActiveRecord models.

This MR adds a reactive_cache_worker_finder which allows classes to define a method that will be called by ReactiveCachingWorker. This method should return an object of the class that includes ReactiveCaching. This allows ReactiveCache to be used by classes that are not ActiveRecord models e.g service classes suche as Prometheus::ProxyService.

What are the relevant issue numbers?

#58375 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer