Add config var to allow disabling automatic CPU group assignment for threads (#38568)

Add config var to allow disabling automatic CPU group assignment for threads (#38568)

- When CPU groups are enabled through config, some new threads (Thread.Start, thread pool threads) are automatically assigned to a CPU group, starting with the process' CPU group and once it is filled up, assigning new threads to a different CPU group. An app may have native components that also use threads, and may want to do its own thread-spreading (for instance in DllMain), this config allows such apps to disable the automatic CPU group assignment which would otherwise override an assignment made in DllMain. The new config var does not affect GC threads.
- This was requested for .NET Framework, porting to .NET Core as well