Publish `H2JUnitTestResultStorage` for conformance to pluggable storage for other plugins to use

### What feature do you want to see added?

It was copied into realtime junit in https://github.com/jenkinsci/junit-realtime-test-reporter-plugin/pull/36

Class:
https://github.com/jenkinsci/junit-realtime-test-reporter-plugin/blob/1af2f977de6a9cff6f085d0a41201b13960e7f5d/src/test/java/org/jenkinsci/plugins/junitrealtimetestreporter/storage/H2JunitTestResultStorage.java

Ideally this would be published in a tests jar.

### Upstream changes

_No response_

Start with H2JunitTestResultStorage.java from the linked realtime test reporter plugin pull request, then inspect this repository’s packaging configuration for how a tests jar could expose it. Done means the storage class is published for other plugins to use while preserving conformance with pluggable storage.

Written by the indexing model from the issue text.