Support multiple --sign-container-identity for cosign sign

[emilyzheng]

Description

If an image can be pulled from multiple references (more than one image proxy or tag), cosign sign has to be run multiple times to include all of them:

cosign sign --sign-container-identity registry1/repo:tag1 <IMAGE DIGEST>
cosign sign --sign-container-identity registry1/repo:tag2 <IMAGE DIGEST>
cosign sign --sign-container-identity registry2/repo:tag1 <IMAGE DIGEST>
cosign sign --sign-container-identity registry2/repo:tag2 <IMAGE DIGEST>

Can cosign sign support multiple container identities so that the command only needs to be run once?

cosign sign --sign-container-identity registry1/repo:tag1 --sign-container-identity registry1/repo:tag2 --sign-container-identity registry2/repo:tag1 --sign-container-identity registry2/repo:tag2 <IMAGE DIGEST>

It would reduce write operations to the same signature and make signing more efficiently in this case.