Support token-based auth in maven_export/MavenPublisher

[SanjayVas]

The maven_export rule and the corresponding MavenPublisher application only support basic auth using username/password. This does not work for repositories which use token-based auth. The most common example of this is GitHub Packages, where uploading via a GitHub Actions is done using GITHUB_TOKEN.

For GitHub specifically there is a workaround using a personal access token (classic), but this effectively means having to authenticate as a single GitHub user with the right permissions.