Add Code Coverage targets

It would be nice to add targets/properties to allow code coverage report generation out of the box in Arcade. Bonus points if it produces a format that AZDO pipelines can ingest by default.

After fiddling with things a bit it seems that the `dotnet exec $(TestRunnerPath)` [method of executing tests](https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Arcade.Sdk/tools/XUnit/XUnit.targets#L51) used by the Xunit target prevents using packages like `coverlet.msbuild`, so an implementation in Arcade or documentation of a simple way to DIY would be ideal.