Extension annotation processor improvements

[gsmet]

This improves compatibility with ZipFileSystems and allows to easily visit a ConfigRoot.

It was extracted from #42677 as it's useful by itself and we want it, no matter what we do with this experiment.

[github-actions]

🎊 PR Preview 09c8742 has been successfully built and deployed to https://quarkus-pr-main-42708-preview.surge.sh/version/main/guides/

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[quarkus-bot]

Status for workflow Quarkus Documentation CI

This is the status report for running Quarkus Documentation CI on commit bdc25a7.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

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[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit bdc25a7.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.