Allow item_task imports from python packages

[kotfic]

Deliverables:

• Importing from a python package with the girder_worker_plugins package will create item_task items for each task that has the self-describing API
• Generated UI for item tasks will run celery tasks with correct inputs etc.

  Technical Notes:

• GirderWorkerPlugin API can be used to list tasks provided by various python packages (e.g. via the girder_worker_plugins) entrypoint
• Tasks can be instantiated and use self-describing API to provide input/output spec for item_tasks

  Depends on:

• implement self-describing Celery task API (https://github.com/girder/girder_worker/issues/143)

[danlamanna]

@kotfic is this still relevant?