Move URLSession uploading logic from UploadFilesOperation to APIClient

[PimCoumans]

UploadFilesOperation should have little to do with creating a new URLSession and its delegate method. Ideally APIClient should have a method to create a new APIClient and accept a closure in which the delegate call can be passed through. These created URLSession instances and linked closures need to be removed when UploadFilesOperation finishes.