IBC scaffold: create a type for the acknowledgment

[lumtis]

Right now we're returning an object channeltypes.Acknowledgement after a packet recv that contains:
channeltypes.NewResultAcknowledgement([]byte{byte(1)}) if packet successful
channeltypes.NewErrorAcknowledgement(err.Error()) if failure

We should scaffold the model of a custom type for acknowledgment for each packet and be able to provide fields with the flag --ack

starport packet foo --ack too,tata:int

Create:

message FooPacketAck {
    string toto = 1;
    int32 tata = 2;
}