Provide an Identity Object in the Workflow to identify User and allow processing of Validation rules inside a Workflow

As mention in discord and during weekly community session, it would be great to embed some kind of `identity object` in the Workflow Context.

Coupled with some kind of activity, this object will allow the Workflow Creator to create some validation rules based on the User Identity (Group/Role/Name etc...)

Some examples could be :
- know who launch a Workflow from the Designer
- identify user or application that launch a workflow exposed using HTTP Endpoint