Add a decrypted field proxy to the model.

        Add to the field model an `EncryptedProxyField` to get the decrypted
        values of the field.

        The decrypted value can be accessed using the field's name attribute on
        the model instance.

def contribute_to_class(self, cls, name, **kwargs):
        """
        Add a decrypted field proxy to the model.

        Add to the field model an `EncryptedProxyField` to get the decrypted
        values of the field.

        The decrypted value can be accessed using the field's name attribute on
        the model instance.
        """
        super(PGPMixin, self).contribute_to_class(cls, name, **kwargs)
        setattr(cls, self.name, self.descriptor_class(field=self))