Add support for configuring Scala Steward in private repositories

[Hector Castro]

We've setup Scala Steward in a few projects, but always using an approach that requires the target repository to be public. For this issue, investigate solutions like scala-steward-org/scala-steward-action to keep Scala dependencies up-to-date for private projects.

(There may even be a way to centrally manage all Scala Steward target repositories.)