Single asset selector.
type AssetField<T extends Record<string, any> = Record<string, never>> = (AssetReference & T) | null;
type FeaturedImage = AssetField<{ caption: string; credits: string }>;
// Result: {
// alt: string | null;
// height: number;
// id: string;
// permalink: string | null;
// width: number;
// caption: string;
// credits: string;
// } | null