Move Saga and Saga operation state variables to be under a single class each to simplify load and store state

[alonf]

Currently the state of the Saga and the Saga operations are in a set of variables. We can group all state variables to a class or a record and then store them in a single key on the state store, meaning that we serialize them only once and not per variable.
It will enable versioning, meaning that in the future, if we change an entry in this record, we can load the old one, and provide a transformation function to the new context, enabling long-running saga to continue even when we update the Sagaway version with a braking change.