The graph representation of a share list is wrong

currently we pul all users with the same share role in a single graph drive item permission as a list of identitysets in the grantedtoidentities property.

This prevents us from using the expirationDate of the permission (which we [still need to add](https://github.com/owncloud/libre-graph-api/issues/86)).

We should create a dedicated permission for every recipient / share role

This might be a breaking change, but I don't know if any client is using this api already.

cc @C0rby @micbar