Support CacheAdapters

[mnightingale]

Summary of the change: Support CacheAdapters

Laravel recommends using CachedAdapter from league/flysystem-cached-adapter to cache driver meta data.

Prior to this commit it wasn't supported by this package and throws the following exception when attempting to upload a file:

Call to undefined method League\Flysystem\Cached\CachedAdapter::getPathPrefix() {"exception":"[object] (Symfony\\Component\\Debug\\Exception\\FatalThrowableError(code: 0): Call to undefined method League\\Flysystem\\Cached\\CachedAdapter::getPathPrefix() at /SNIP/vendor/unisharp/laravel-filemanager/src/LfmStorageRepository.php:29)
[stacktrace]
#0 /SNIP/vendor/unisharp/laravel-filemanager/src/LfmPath.php(77): UniSharp\\LaravelFilemanager\\LfmStorageRepository->rootPath()
#1 /SNIP/vendor/unisharp/laravel-filemanager/src/LfmPath.php(222): UniSharp\\LaravelFilemanager\\LfmPath->path('absolute')
#2 /SNIP/vendor/unisharp/laravel-filemanager/src/Controllers/UploadController.php(34): UniSharp\\LaravelFilemanager\\LfmPath->upload(Object(Illuminate\\Http\\UploadedFile))
#3 [internal function]: UniSharp\\LaravelFilemanager\\Controllers\\UploadController->upload()

The fix in this PR is similar to as used in laravel/framework at https://github.com/laravel/framework/blob/5.7/src/Illuminate/Filesystem/FilesystemAdapter.php#L376-L397 in that it unwraps the CachedAdapter.

It has made a significant improvement to my file listing performance with S3.

[streamtw]

We notice there is some serious performance issue with S3 when integrating laravel file system, just cannot figure why. So glad to receive this PR! Thanks @mnightingale !