Add RetrieveOrCreateMachineAsync to IStateEngine`2

[aidapsibr]

It is difficult to handle the case where a caller may need to either retrieve the machine or create it if it does not already exist. Currently it involves no less than 2 try catch blocks for an end consumer to ensure they successfully get a machine.

[aidapsibr]

This should be mirrored in INaturalStateMachine as well.