Support Saga in Go SDK

Pretty self-explanatory. There is an open issue about this in Cadence Go SDK as well (and a PR that fell through).

I believe the recommendation is to have an interface as follows (similar to the Java SDK, but with ctx to pass the saga):

sagaCtx := saga.New(ctx, options)
saga.addCompensation(sagaCtx, ....)

And utilising Workflow.NewDisconnectedContext behind the scenes to ensure the compensation code continues to execute even after the root context has been cancelled.