Support `rules_jvm_external` or `j2cl_maven_import_external` private registry

Hi 👋
I would like to use `j2cl` with private maven registry. I checked `j2cl_maven_import_external` function, but it is only support public repository download: https://github.com/google/j2cl/blob/54deeb5ea6913c2c6d256a7ea80d3fff4d614d44/build_defs/internal_do_not_use/j2cl_repo.bzl#L36
It would be great add credential support to download method: https://github.com/bazelbuild/bazel/blob/290fc80a5aae9dea06de52deed7098a5b8443f26/src/main/java/com/google/devtools/build/lib/bazel/repository/downloader/HttpDownloader.java#L77

Maybe an other solution when it would be possible integrate with `rules_jvm_external` rule, which support private registries.
https://github.com/bazelbuild/rules_jvm_external#features

Thank you very much for the response.