Get or set the AssetBundle variant.
AssetBundle variant is combined with the AssetImporter.assetBundleName as the file extension to generate the full AssetBundle name.
AssetBundle variant is used to achieve virtual assets via AssetBundle. AssetBundles which have the same AssetBundle name but different AssetBundle variants will have the same internal IDs. So they can be switched out arbitrarily with AssetBundles of different variants.
Please make sure the assets exactly match in variant AssetBundles.