Introduce multithreaded task execution capability #12461
Description
Create thread-safe MSBuild nodes that lives within the main process to enable concurrent task execution. Tasks that are not thread-safe (which on this stage expected to include most existing tasks) will run in compatibility mode via isolated task host processes. Ensure correctness of the builds.
Goal: MSBuild is able to build any solution in multithreaded mode.
Out of scope: Extensive testing for thread-safety issues.