Move EventPipe env var parsing to native code

[sywhang]

Today EventPipe's logic for handling the environment variable COMPlus_EnableEventPipe is written entirely in managed code, which means that using this does not give us some of the very early events from the runtime startup.

There are some interesting runtime events that would be desirable to be tracked before managed code runs, so it would be a good idea to move the parsing logic to native code.

LTTng already has prior art in doing this, so this shouldn't be a huge work item.

This work item is part of enabling dotnet/diagnostics#619.

[sywhang]

Fixed with #32516