Build ThirdPartyResource API group

[munnerz]

We need to begin creating an API group to hold the contents of the old stable.k8s.psg.io/v1 group.

We can automatically generate the API group for a single types.go file, like in other repositories that have implemented ThirdPartyResources. We've got an example of this over in our navigator repo here: https://github.com/jetstack-experimental/navigator/tree/master/pkg/apis/navigator

Based on the naming of the service-catalog API group, I propose certmanager.k8s.io be used.

[munnerz]

I've built out an API now corresponding of Issuers and Certificates - have a look over it in pkg/apis/certmanager!

There's some example manifests in docs/ too.