* Add a configure option to use a locally built dotnet/runtime.
* Add documentation how to build dotnet/runtime the way we need it built.
* Modify our build to consume the custom dotnet/runtime if so configured.
This is useful when trying to debug the runtime locally, or trying out new
features there are no packages for yet.
Co-authored-by: TJ Lambert <50846373+tj-devel709@users.noreply.github.com>