add `--image-verify` option for verifying container images

[hirokuni-kitahara]

Signed-off-by: Hirokuni-Kitahara1 hirokuni.kitahara1@ibm.com

• when --image-verify flag is enabled in verify-resource command, the command verifies not only the specified resources but also container images of those resources.

Example:

$ kubectl sigstore verify-resource deployment sample-app --image-verify

...

[RESOURCES]
KIND         NAME         VALID   ERROR   AGE
Deployment   sample-app   true            5d

[RESOURCES - PODS/CONTAINERS]
POD                          CONTAINER    IMAGE ID                     SIGNED   SIGNER
sample-app-b494cd9c8-m7lmz   nginx        nginx:latest@IMAGE_DIGEST    true     sample-signer@example.com