remove dependency on org.reflections

[jsvd]

This removes the annoying log info:

[2025-10-14T16:46:35,085][INFO ][org.reflections.Reflections] Reflections took 44 ms to scan 1 urls, producing 149 keys and 523 values

Given we only use reflectiong to grab locally installed java plugin classes by annotation, we implement here a tiny scanner to do the same, and then we can remove the dependency.

exhaustive test suite run: https://buildkite.com/elastic/logstash-exhaustive-tests-pipeline/builds/2701/steps/canvas

[github-actions]

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label. Could you fix it @jsvd? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 26942ce

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• 🥼 IT Persistent Queues / part 4-of-6