implement middleware to impersonate a user for presigned urls

[butonic]

clients might be able to create pre signed urls with a random key generated using ocs: owncloud/ocis-ocs#18

the proxy needs to be able to authenticate a request, in a similar fashion as for oc10 owncloud/core#37634

if we talk directly to ocis-store we would bypass ocis-ocs, who is responsible for the data. instead we need to add an rpc call to ocis-ocs to validate the signature of the request.

If done properly this is an independent service, similar to the reva auth basic, aith bearer or auth publictoken auth service instances that are responsible for authenticating requests.