Allow discovery of teams configuration files in subfolders

[JoshuaTheMiller]

• Perhaps use a configurable glob style syntax for discovery?
  • https://docs.microsoft.com/en-us/dotnet/core/extensions/file-globbing#get-all-matching-files
• Must flatten full list to validate no duplicates
  • If duplicate detected, fail operation
  • Failure of an operation would be reported either by the Action or the GitHub App as a commit/PR status. Do not embed reporting logic in Core

Why allow this? To leverage CODEOWNERS as a means to delegate trust for team creation