add-team-to-repo should support custom repository roles

This is a relatively new feature in github (public beta):
https://docs.github.com/en/enterprise-cloud@latest/organizations/managing-peoples-access-to-your-organization-with-roles/managing-custom-repository-roles-for-an-organization

The add-team-to-repo command validates that the role is one of the 5 non-custom roles in GitHub. This validation will need to be loosened to support custom roles and tested appropriately.