Enhancement: Add OCM scope and webdav endpoint

Adds the OCM scope and the ocmshares authentication,
to authenticate the federated user to use the OCM shared
resources.
It also adds the (unprotected) webdav endpoint used to interact with
the shared resources.

https://github.com/cs3org/reva/pull/3691
https://github.com/cs3org/reva/issues/2739