properties_path of BundleWorkflow only support json format

[yiheng-wang-nv]

It's different from the default setting: using a .py file to save properties:

MONAI/monai/bundle/workflows.py

Line 51 in 684688a

properties_path: the path to the JSON file of properties.

For users that want to define a properties file, a simple way is to copy and modify the file: https://github.com/Project-MONAI/MONAI/blob/684688a2877b07b0e1da98bb13f18a60ac73d426/monai/bundle/properties.py

it's inconvenient to use json since it requires to manually "translate" ID_SEP_KEY, BundleProperty and BundlePropertyConfig.

[yiheng-wang-nv]

cc @Nic-Ma @KumoLiu