Process validation on node schema when completing a task

• a transient DocumentModel will be created associated with the node schema handling the local variables + the transient schema handling the global variable used within the form
• From the variables map (workflow + node) submitted for task completion, the 2 schema fields will be valued
• if the wokflow variable map contains a variable that does not match one of the field of the transient schema, a security exception is thrown (this means we are trying to edit a global variable within a task completion that we are not allowed to)
• the validation is processed on this DocumentModel but nothing is persisted.
• if the validation went through, the workflow engine will complete the task and run as it is

Thanks to NXS-2442, we will have a single node schema which will hold fields associated to node variables along with their validation constraints.

We'll use it to validation task completion.