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

This issue is to follow the discussion in PR https://github.com/dapr/java-sdk/pull/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{
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.