Is your feature request related to a problem? Please describe.
Currently, we have the monai.bundle.download() API to automatically download the bundle from MONAI model-zoo or direct URL. Would be nice to add the NGC source support to this function.
And also use a system environment to control the default source, for example changing the default arg to:

source: str = os.environ.get("BUNDLE_DOWNLOAD_SRC", "github")

If source is not github or ngc and no direct URL provided, raise ValueError.
Better to include this feature in MONAI 1.1 in case we need it soon.