Fill ThreadPool command data from portable thread pool info when enab…

…led (#1274)

Related to and depends on dotnet/runtime#38225

When the managed portable thread pool is enabled:
- Made the `ThreadPool` command work as expected, including `ThreadPool -ti` to show in-memory hill climbing thread adjustment history
- After the command queries the native side for info, it looks for a couple of static variables to determine if the portable thread pool is available and enabled
- If it's enabled, it collects equivalent information from the managed side
- Verified that the command works with and without the changes in dotnet/runtime#38225, and with the changes in both modes (portable thread pool enabled and disabled)