[OPR-152]: Adds overload for addObserver
[OPR-152]: Adds overload for addObserver

ProcedureKit/ProcedureKit,cezarywojcik/Operations,ProcedureKit/ProcedureKit,danthorpe/Operations,danthorpe/Operations,danthorpe/Operations,blg-andreasbraun/Operations,blg-andreasbraun/ProcedureKit,cezarywojcik/Operations,ProcedureKit/ProcedureKit,blg-andreasbraun/ProcedureKit,cezarywojcik/Operations,blg-andreasbraun/ProcedureKit,blg-andreasbraun/Operations,blg-andreasbraun/Operations