No way to pass user data to thread service callback

Customarily, callbacks take an additional user data parameter so that closures can be used as callbacks. This is the case with, for example, `JsMemoryAllocationCallback`. However, `JsThreadServiceCallback` takes no such parameter (do not confuse this with its `callbackState` parameter, which is not _user_ data rather engine-provided data for the work item callback).

Ideally the API would be:

```c
typedef bool (CALLBACK *JsThreadServiceCallback)(
_In_ JsBackgroundWorkItemCallback workItemCallback,
_In_opt_ void *workItemCallbackState,
_In_opt_ void *threadServiceCallbackState
);

STDAPI_(JsErrorCode) JsCreateRuntime(
_In_ JsRuntimeAttributes attributes,
_In_opt_ JsThreadServiceCallback threadService,
_In_opt_ void *threadServiceCallbackState
_Out_ JsRuntimeHandle *runtime
);
```

where the runtime would pass `threadServiceCallbackState` to `threadService` whenever it’s called.