Provider fake load-balancer service implementation

Similar to our [fake tag service](https://github.com/digitalocean/digitalocean-cloud-controller-manager/blob/442aa00e4b84a470298488a055b54d7ee4d6c1b7/cloud-controller-manager/do/faketagssvc_test.go), we should also provide a fake load-balancer service. This will make testing much simpler because testers won't need to think about what each API method should return anymore. Instead, it allows to focus on what load-balancer resources the fake should be populated with initially and leave the rest to the implemented logic.