Design: use structured response data

[alpe]

Background: simple way to access the completion time in confio/tgrade-contracts#152

As a client working with a contract, I can only trust the data field to come from the contract that I am calling. Events are unstructured data and may originate from a subcall so that I need to check for the origin of the event. To parse data from a wasm event, I would have to do in tgrade:

	// parse events for unboundTime
	for _, e := range em.Events() {
		if e.Type != wasmtypes.WasmModuleEventType {
			continue
		}
		var trusted bool
		for _, a := range e.Attributes {
			if string(a.Key) != wasmtypes.AttributeKeyContractAddr || string(a.Value) != contractAddr.String() {
				continue
			}
			trusted = true
			break
		}
		if !trusted {//  filter out other events
			continue
		}
		for _, a := range e.Attributes { 
			if string(a.Key) != "completion_time" {
				continue
			}
			// here I parse the data from the raw string representation
			nanos, err := strconv.ParseInt(string(a.Value), 10, 64)
			if err != nil {
				return nil, err
			}
			completionTime := time.Unix(0, nanos).UTC()
			return &completionTime, nil
		}

	}
	return nil, types.ErrInvalid.Wrap("completion_time not found")

compared to unmarshaling structured json data from the response data field this is quite some overhead

We have this model of structured response data already for queries/ responses and it works well for me.

Note: with submessages the data field could also be overwritten but I would consider the contract to be in charge and taking care as it is orchestrating the workflow.