TypeScript - externalManifest is optional for several methods.

[matvp91]

Found a few TS overrides in my code, they're better off being fixed upstream.

// Types that are missing or wrong in dashjs, should push this upstream too.
declare module "dashjs" {
  class DashAdapter {
    getAvailabilityStartTime(externalManifest?: object): number;
    getIsDynamic(externalManifest?: object): boolean;
    getDuration(externalManifest?: object): number;
  }
}