Add reference to public header

Thank you for this awesome project. I am using the [add_file_references](https://www.rubydoc.info/github/CocoaPods/Xcodeproj/Xcodeproj/Project/Object/PBXNativeTarget:add_file_references) to add the references to header/implementation files. The add_file_references method adds the reference to the file successfully but how can add a public reference to a header file. The add_file_references only adds reference to files with Project access control.

Start at Xcodeproj::Project::Object::PBXNativeTarget#add_file_references, the entry point named in the issue, and inspect how it assigns project access control to header and implementation references. Done means the API supports adding a header with public access control, with behavior verified against the project's existing tests.

Written by the indexing model from the issue text.