create PR validation that checks whether submitted user is specified in model_metadata

[nickreich]

Based in part on suggestion from collaborators in Italy. Suggested that model_metadata could have github users specified who are the only people who could contribute files for a specific model to a hub.

[bsweger]

Could something like this be accomplished with GitHub's code owners feature? https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners