Automatic merge from submit-queue. 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>.

apiextensions: add optional comment tags

Adds `// +optional` comment tags to fields which are optional/omitempty for CRD types.

These tags are used for openapi generation:

https://github.com/kubernetes/kube-openapi/blob/0cf8f7e6ed1d2e3d47d02e3b6e559369af24d803/pkg/generators/openapi.go#L361-L362

https://github.com/kubernetes/kube-openapi/blob/0cf8f7e6ed1d2e3d47d02e3b6e559369af24d803/pkg/generators/openapi.go#L81-L89

**Release note**:

```release-note
NONE
```