Add workflowInstanceDataFields and others nodes for API Tasks method

Currently, in 5.8, it's not possible to get the list of Datafield in the current operation 'Tasks'

Step to see it:

A) Activate Workflow REST

B) Go to this URL: https://nintex.k2test.com/api/workflow/preview/swagger/index#!/Tasks

or

https://nintex.k2test.com/Api/Workflow/Preview/tasks

Result: The node 'workflowInstanceDataFields' is not present: only actions and systemActions are available

Expected result: Other node 'workflowInstanceDataFields' should be here.

Note: we can add too 'activityDataFields', 'workflowInstanceXmlFields','itemReferences' and maybe 'workflowInstanceDataFieldsString' that we have with get /tasks/{serialNumber}

Example: https://nintex.k2test.com/Api/Workflow/Preview/tasks/39_43