Automatic merge from submit-queue (batch tested with PRs 53317, 52186). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

code-generator: rewrite hack/update-codecs.sh into reusable generate-{internal,}-groups.sh

Generating everything for groups inside of an apiserver (with internal types) becomes:

```shell
generate-internal-groups.sh all "$(dirname ${BASH_SOURCE})/../../.." k8s.io/sample-apiserver/pkg/client k8s.io/sample-apiserver/pkg/apis k8s.io/sample-apiserver/pkg/apis wardle:v1alpha1
```

Generating everything for a CRD (versioned types) becomes:

```shell
generate-groups.sh all "$(dirname ${BASH_SOURCE})/../../.." k8s.io/sample-apiserver/pkg/client k8s.io/sample-apiserver/pkg/apis wardle:v1alpha1
```

This should cover the 90% percent use-case. For the other 10% this can be forked and adapted as needed.

Furthermore, we can put this into a Docker container. Then code-generator consumers can do:

```shell
$ docker run -v $GOPATH:/go k8s.io/code-generator:1.8 generate-group.sh github.com/foo/bar example:v1
```

This is only possibly only the first step towards a `code-generator` binary. For the later deeper generator changes are necessary (e.g. https://github.com/kubernetes/kubernetes/pull/53202) and hence the later is only feasible in 1.9. **This PR here in contrast, we can cherry-pick to 1.8.**

```release-note
Add generate-groups.sh and generate-internal-groups.sh to k8s.io/code-generator to easily run generators against CRD or User API Server types.
```

Fixes https://github.com/kubernetes/kubernetes/issues/48714.