nnUNet download option for available pretrained models

Summary:
nnUNet pretrained models can now be downloaded from MITK. This feature is implemented in a separate non-blocking thread with option to interrupt.
Ref: T29187, T29199

Test Plan: NA

Reviewers: kislinsk, kalali, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Differential Revision: https://phabricator.mitk.org/D671