allow to add uuid as id to proxies

As described in:
https://gist.github.com/ElectricMaxxx/68a7d033a6357e757329
i would wish to create proxies (after calling `$dm->getReference('MyClassName', 'uuid');`) with the uuid as the id. This PR will make it possible that the request on the backend is able to handle the uuid. 
Normally `$session->getNode($id);` will fetch the node. Now there will be a check if the `$id` is an uuid and fetch the node by `$session->getNodeByIdentifier($id);` , which is able to look for a node by its uuid.

I tried to create more tests for those documents that live as proxies in the UoW, but i do not get it to solve the mocking and i do not know if it is clever to do so, cause if i would mock some implementation depending, we would lose the decoupled view on the backend.