Added support for collection initializer enabling more convenient syntax

var sf = new SagaFixture(...)
sf.AddSagaData(new SagaData());
becomes:
new SagaFixture(){ new SagaData()}
Also expose AvailableSagaData as IEnumerable on the class (partly to support the collection initializer pattern, and maybe avoid some demeter)

Dunno if AddSagaData should be made obsolete and the rest of the overloads supported aswell?