Implement missing features for multi-cluster client-go

[xrstf]

Since refactoring the code-generator to v3, we lost some CLI flags that kcp's client-go is using to generate code: https://github.com/kcp-dev/client-go/blob/kcp-1.32.3/hack/update-codegen.sh effectively does (formatted for readability):

${CODE_GENERATOR} \
client
  externalOnly=true
  standalone=true
  outputPackagePath=github.com/kcp-dev/client-go
  name=kubernetes
  apiPackagePath=k8s.io/api
  singleClusterClientPackagePath=k8s.io/client-go/kubernetes
  singleClusterApplyConfigurationsPackagePath=k8s.io/client-go/applyconfigurations
  headerFile=./hack/boilerplate/boilerplate.go.txt
lister
  apiPackagePath=k8s.io/api
  singleClusterListerPackagePath=k8s.io/client-go/listers
  headerFile=./hack/boilerplate/boilerplate.go.txt
informer
  clientsetName=kubernetes
  externalOnly=true
  standalone=true
  outputPackagePath=github.com/kcp-dev/client-go
  apiPackagePath=k8s.io/api
  singleClusterClientPackagePath=k8s.io/client-go/kubernetes
  singleClusterListerPackagePath=k8s.io/client-go/listers
  singleClusterInformerPackagePath=k8s.io/client-go/informers
  headerFile=./hack/boilerplate/boilerplate.go.txt
"paths=$( go list -m -json k8s.io/api | jq --raw-output .Dir )/..." \
  "output:dir=./"

I only got so far that client.externalOnly, client.standalone and client.apiPackagePath are probably features/flags we do not yet support in v3. This means in our client-go repository we currently still have to use the v2 generator.