Ambiguous `result` element

It is hard and untidy to consume the `result` element, because in context of `status: PENDING` it is a string/URL, and in context of `status: ok`, it is a structure. I recommend putting the URL to a distinctly different element outside of the `result` field.

Here is my understanding, of what the returned json body semantic:
https://github.com/jan-dolejsi/pddl-planning-service-client/blob/main/src/PlannerPackagePreviewService.ts#L125

Consider this:

```json
{
"status": "PENDING",
"check": "/check/......."
}
```

... vs ...

```json
{
"status": "ok",
"result": {
}
}
```