Miscellaneous improvements related to Durable Task Hub

[MicroFish91]

Summary:

• Improve how the task hub shows up in the tree view (Add DTS hub icon & show scheduler name as part of the label)

• Assign DTS role to the task hub instead of the scheduler (principle of least privilege)
• Move role assignment logic from scheduler list step to task hub list step
• Consolidate / add an activity child during the role assignment callback so users can see which managed identity has the role if it already exists