Need utility to map strings to our Enum args

[Nic-Ma]

Is your feature request related to a problem? Please describe.
Currently, many APIs take Enum args from: https://github.com/Project-MONAI/MONAI/blob/master/monai/utils/enums.py.
But we want to call the function from strings, need a mapping tool or something for it.

[Nic-Ma]

I will check all the call of Enum objects, make sure all of them can support string.

Thanks.