Allow extraHeaders for fetchNavigation

In a new version the navigationService can be secured with a token authentication. In order to access it from the fsxa-api "fetchNavigation()" function, it should be possible to pass additional headers, e.g.
```
async fetchNavigation(
    initialPath: string | null,
    defaultLocale: string,
   extraHeaders?: Record<string, string[]>
  ): Promise<NavigationData | null> {
```