Add support for serialization of `astropy.nddata.StdDevUncertainty` and `astropy.nddata.UnknownUncertainty` objects.

Implement the converter class for the below classes which are subclasses of `astropy.nddata.NDuncertainty`

- Implement converter and design schema for `astropy.nddata.nduncertainty.StdDevUncertainty` .

- Implement converter and design schema for `astropy.nddata.nduncertainty.UnknownUncertainty`.