Service response values are not unwrapped

As betterproto is aiming to support automatic (un)wrapping of well known types (google wrapper values), the service responses should also return the underlying values, instead of the wrapper objects.

```protobuf
syntax = "proto3";

import "google/protobuf/wrappers.proto";

// Tests that wrapped values can be used directly as return values

service Test {
rpc GetDouble (Input) returns (google.protobuf.DoubleValue);
}

message Input {

}
```

```python

response = TestStub(channel).get_double()

assert type(response) == float # fails!
```