fetchPolicy and nextFetchPolicy have fixed values for PreloadQuery

When using the PreloadQuery, you will get a TypeScript error if you're trying to set the value of `fetchPolicy` to something other than `cache-first`.
`nextFetchPolicy` is typed to always be `undefined`.

```typescript
type RestrictedPreloadOptions = {
fetchPolicy?: "cache-first";
returnPartialData?: false;
nextFetchPolicy?: undefined;
pollInterval?: undefined;
};
type PreloadQueryOptions = QueryOptions & RestrictedPreloadOptions;
```

Is this intended?

Start at the RestrictedPreloadOptions and PreloadQueryOptions definitions shown in the issue, then trace how PreloadQuery consumes QueryOptions and its fetch-policy types. Determine which policies are intended to be supported; done means the TypeScript types match that decision and the relevant type checks cover fetchPolicy and nextFetchPolicy.

Written by the indexing model from the issue text.