Use generic and the serializer interface to return the actual object for workflow activity

[skyao]

This issue is to follow the discussion in PR #880

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artursouza:
Use generic and the serializer interface to return the actual object. See how it is done for DaprClient

skyao:
this run() method defined in WorkflowActivity doesn't have a input parameter, we need to call ctx.getInput() to get the input, like this:

public Object run(WorkflowActivityContext ctx) {
    OrderPayload order = ctx.getInput(OrderPayload.class);
    logger.info("Requesting approval for order: {}", order);

    return ApprovalResult.Approved;
}

I suggest to move this input parameter from ctx to run() method:

public interface WorkflowActivity<Input, Output>{
Output run(WorkflowActivityContext ctx, Input input);
}

artursouza: Exactly. Also, use TypeRef from the main SDK. See how we do it for Actors and state store.

[cgillum]

This is a breaking change, so if it needs to be done, it needs to be done in v1.15.