Allow comparing extension changes with uninstalled versions

It would be great to be able to compare an installed extension (thus in a given version), with another version (not installed yet), as a way to know the potential problems before deciding to upgrade (or not).

Discussion here: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$15609439915857iGkDB:matrix.xwiki.com?via=matrix.xwiki.com&via=matrix.org&via=chat.weho.st