extend storage api to allow directly writing a stream to storage

this removes the need for temporary storages with some external storage backends.
The new method is added to a separate interface to maintain compatibility with
storage backends implementing the storage interface directly (without inheriting common)

Currently the interface is implemented for objectstorage based storages and local storage
and used by webdav uploads

Signed-off-by: Robin Appelman <robin@icewind.nl>