StorageService add getBucketFolders & refactor

I needed an API call that receives all the folders in one flat get request without any files. I thought it would be better to do this then make a lot of .get requests recursively on the client every time you need a folder list.
I suddenly found that I could use the same code into a private function and have the function return a custom class with two arraylists, one for all Items from the root and one with all Folders.  I created the BucketItems class and had the private functions return that.
add folders.get API call
fixed spacing & add folders array list
added test for getFolders service method
add BucketItems class used by StorageService
add e2e tests for folders.get API