picobuf compatibility with regards to Marshal and Unmarshal.

[egonelbre]

At the moment picobuf.Marshal and picobuf.Unmarshal expect messages of type picobuf.Message, which has some specific methods. However, drpc endpoints create typedefinitions such as:

type drpcEncoding_File_certificate_proto struct{}

func (drpcEncoding_File_certificate_proto) Marshal(msg drpc.Message) ([]byte, error) {
	return picobuf.Marshal(msg)
}

func (drpcEncoding_File_certificate_proto) Unmarshal(buf []byte, msg drpc.Message) error {
	return picobuf.Unmarshal(buf, msg)
}

func (c *drpcCertificatesClient) Sign(ctx context.Context, in *SigningRequest) (*SigningResponse, error) {
	out := new(SigningResponse)
	err := c.cc.Invoke(ctx, "/node.Certificates/Sign", drpcEncoding_File_certificate_proto{}, in, out)
	if err != nil {
		return nil, err
	}
	return out, nil
}

I would expect the Invoke to register with a more specific message such as picobuf.Message instead of relying on drpc.Message and expecting that Marshal and Unmarshal don't have additional constraints.