validators/wikimedia: Add support for Private-Change footer
The intention is to use Change-private in the puppet repo as a way of running pcc using a proposed change to the labs private repo
Related-Id: I614aa7bd362c6484600a59fb02548f1c91c1f7c2
Related-Id: I96c594779afbd444e5b14fe546984aec50c6a2d8
Related-Id: I614aa7bd362c6484600a59fb02548f1c91c1f7c2
Bug: T265633