`dotnet format` support for traversal / .proj files

### Is your feature request related to a problem? Please describe.
I want to use https://github.com/leonardochaia/dotnet-affected as the target of my `dotnet format` command so I don't waste PR CI time on formatting files that are unchanged and unaffected by the PR.
Currently `dotnet format` only supports .csproj files and .sln files, but `dotnet affected` outputs a .proj file.

### Describe the solution you'd like
That .proj traversal file be supported by `dotnet format`.
It is already supported by `dotnet build` and `dotnet test`.