Add publishStaging to sbt

[xerial]

Started a discussion at:
https://discuss.lightbend.com/t/idea-add-publishstaging-task-to-sbt-for-creating-bundles-for-sonatype-upload/5020

Related:

• SBT may publish to Maven Central lot more efficiently sbt/sbt#4958

[mgajda]

Indeed, I get 401 Unauthorized on staging repo.
It would be better to at least get a reasonable error message.