[App CR] Support signature verification for fetched artefacts using Sigstore

[ThomasVitale]

Describe the problem/challenge you have
When fetching artefacts as part of the App CR, it would be great if it was possible to verify their signature in advance (Git commits, OCI images...). This could be added once vendir gets the Sigstore integration as per carvel-dev/vendir#92.

There's a similar issue about doing the same, but only for Git commits when using GPG: #6.

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[neil-hickey]

Yes! We have been thinking about this for a while, and it's coming up as a strong 'yes please' from the community. As you mentioned it's got to be implemented in imgpkg first, so I will 'untriage' this issue, and leave it open so we can update it when this work is scheduled, thanks as always @ThomasVitale !