A simple feature sink which proxies feature addition on to another feature sink.
This class is designed to allow factory methods which always return new QgsFeatureSink
objects. Since it is not always possible to create an entirely new QgsFeatureSink
(e.g. if the feature sink is a layer or a layer's data provider), a new
QgsProxyFeatureSink can instead be returned which forwards features on to
the destination sink. The proxy sink can be safely deleted without affecting
the destination sink.
