Enhancement: add new file url of the app provider to the ocs capabilities

We've added the new file capability of the app provider to the ocs capabilities, so that
clients can discover this url analogous to the app list and file open urls.

https://github.com/owncloud/ocis/pull/2884
https://github.com/owncloud/ocis/pull/2907
https://github.com/cs3org/reva/pull/2379
https://github.com/owncloud/web/pull/5890#issuecomment-993905242
