Add Prefix utilization for API

[empusas]

I have a request to add the prefix utilization to the API and I have already written a small plugin to enhance the API with this and other features. But I thought that might be useful for others as well to avoid multiple calls to the API.

It would require to add the following code to the prefix serializer as the function is already in the model for the prefixes:

utilization = serializers.SerializerMethodField('get_utilization')

def get_utilization(self, obj):
    utilization = obj.get_utilization()
    return utilization