Codegen Go-native tool

[cardil]

Rewrite codegen-library.sh script into a Golang tool knative.dev/toolbox/codegen. The current script:

https://github.com/knative/hack/blob/158898889ae8670c7e68e522e32cd6800e1729a0/codegen-library.sh#L19-L35

Invent new CLI, for example:

go run knative.dev/toolbox/codegen@latest k8s \
  "deepcopy,client,informer,lister" \
  knative.dev/serving/pkg/client knative.dev/serving/pkg/apis \
  "serving:v1 serving:v1beta1 autoscaling:v1alpha1"

go run knative.dev/toolbox/codegen@latest knative \
  "injection" \
  knative.dev/serving/pkg/client knative.dev/serving/pkg/apis \
  "serving:v1 serving:v1beta1 autoscaling:v1alpha1"

go run knative.dev/toolbox/codegen@latest deepcopy-gen \
  -i knative.dev/serving/pkg/apis/config \
  -i knative.dev/serving/pkg/reconciler/route/config \
  -i knative.dev/serving/pkg/autoscaler/config/autoscalerconfig \
  -i knative.dev/serving/pkg/autoscaler/scaling \
  -i knative.dev/serving/pkg/deployment \
  -i knative.dev/serving/pkg/gc

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[cardil]

/reopen
/remove-lifecycle stale
/triage accepted

[knative-prow]

@cardil: Reopened this issue.

In response to this:

/reopen
/remove-lifecycle stale
/triage accepted

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