Update license headers in new files

[am11]

Second commit is just formatting of the .cs files modified in first commit (based on the rules defined in .editorconfig at repo's root):

$ # dotnet tool install -g dotnet-format --version 5.0.* --add-source https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json
$ git show --name-only --pretty="" :/*.cs | dotnet format --folder --include -

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

Thanks.