Configuration: SD should be an int

The SD value in AMF SNSSAI configuration is a string, but it should be an int. yaml-cpp is able to correctly read also a hex value here, so we can safely use an int type here.

Also, there is no validation of this value.

In any case, I am working on unifying the slicing configuration over all NFs including AMF, so we don't have to fix this in AMF right now.