accept a closure in RunAsStep

[maxdml]

This version of a more flexible RunAsStep interface accepts a closure which only requires a context as first argument:

type GenericStepFunc[R any] func(ctx context.Context) (R, error)

Example usage:

stepCtx, stepCancelFunc := WithTimeout(ctx, 1*time.Millisecond)
defer stepCancelFunc() // Ensure we clean up the context
_, err := RunAsStep[string](stepCtx, func(context context.Context) (string, error) {
    return waitForCancelStep(context)
}) 

The part that's tricky is that we can't use the outer scope context in the step (here, the call to waitForCancel)

This is because we do some context augmentation inside DBOS. And we need to pass our new context to the function.