Experimental: provide functional interface/struct to define workflow

[longquanzheng]

This is a comment/idea from a Temporal user:

The natural way to build abstractions in Go is to use higher order functions instead of OOP style classes-like structs and interfaces (the latter of course have their place but more for the consumer than producer side)
So, e.g. instead of implementing a "class" (struct) per state you could have something like this

var sm iwf.StaMa[StateType] := iwf.NewStaMa("sm-id", initialState, iwf.WithPersistanceOptions(...))

sm.AddState("state-id-1", func(...) {
 // some logic
 return "the-next-state"
}, iwf.WithSomeOption(...))