Support configuring public keys for verifying signature

[andaaron]

[andaaron]

In progress, @Andreea-Lupu to provide the PR.

[andaaron]

Also document how to upload the files/keys/certificates needed to verify the signatures (for both cosign and notation)

[andaaron]

Work is done for local storage. AWS/S3 alternatives are still WIP as part of #1485

[andaaron]

AWS/S3 alternatives are still WIP separate from #1485

[andaaron]

Fixed in #1744