Possibility to extend broker responses

[indvd00m]

In some cases we need to extend OSB specification and add extra fields to responses from service broker. For example it is not possible right now write something like this:

public class MyCreateServiceInstanceResponse extends CreateServiceInstanceResponse {

    private final String myCustomField2;

    ...
}

and after that use this reponse in ServiceInstanceService:

public class MyServiceInstanceService implements ServiceInstanceService {
...
    @Override
    public Mono<MyCreateServiceInstanceResponse> createServiceInstance(CreateServiceInstanceRequest request) {
...
}

Java compiler throw exception like this in that case:

java: my.package.MyServiceInstanceService is not abstract and does not override abstract method createServiceInstance(org.springframework.cloud.servicebroker.model.instance.CreateServiceInstanceRequest) in org.springframework.cloud.servicebroker.service.ServiceInstanceService

This changes are intended to allow this to be done.

[royclarkson]

I started reviewing this, and at first glance, it will cause some challenges with regard to generic types in method parameters and return types. I'll see where I can get with it, but my preference is to avoid significantly disrupting the API.