[Remote validation] Handle the case where the validator OID is not provided

In some cases, and especially when the tool will be called by the proxy, the OID of the cross validator will not be known by the calling tool. It is then the responsibility of the user to select it.
In that case, we cannot "attach" the received files to the input immediately upon receipt of the request.
That means that:
* The reference to the files shall be stored in the bean temporary
* We might want to display to the user how many files have been received
* When the user select a validator in the drop-down list, the files are linked to the correct validator inputs