csi: reformat external volumes objects

[hajiler]

Fixes #2407, #2436.

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR

What changes

One rename in the volume API, split out of #2567 so the breaking part lands before GA and the shared-volume feature can follow as a non-breaking addition.

Volume.external_volume_template becomes Volume.external, typed ExternalVolumeSource, with the existing ExternalVolumeTemplate under its template member. The field name now matches its siblings (durable_dir, system_info, image) and the message follows the *VolumeSource convention (#2407, #2436). The ExternalVolumes preview gate moves onto the new member, so gating behaviour is unchanged. ExternalVolumeSource is a union with one member today; the shared-volume reference is added as a second member in #2567.

volumes:
- name: data
  external:
    template:
      capacity: 10Gi
      storageClassName: standard

The per-actor record in ActorStatus gets wrapped in a new ExternalVolumes message. This will make changes for shared volume support easier post GA.

Callers, tests, demos, benchmark manifests and docs are updated; generated code is regenerated.

Testing

Unit, store (Postgres), controlapi and functional suites pass; hack/verify/gofmt.sh, hack/verify/boilerplate.sh, apitool validate and its exemption test pass.

Breaking change

ActorTemplate.volumes[].externalVolumeTemplate is gone; templates must use volumes[].external.template. Field number 3 is retired, not reused, so templates stored under the old field decode with no volume source and must be recreated (they fail validation on their next update). Go consumers of pkg/proto/ateapipb see the new ExternalVolumeSource type and the moved getter.

Requesting the breaking-change label.

🤖 Generated with Claude Code

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.