Support writing signature and certificate to files #100
Description
Description
We should support the ability to write the certificate and signature(s) to files.
Cosign does this via --output-certificate and --output-signature:
$ COSIGN_EXPERIMENTAL=1 cosign sign-blob --help
Sign the supplied blob, outputting the base64-encoded signature to stdout.
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--output-certificate string write the certificate to FILE
--output-signature string write the signature to FILE
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