feat: verify-blob --cert base64

[caarlos0]

Summary

#1016 added a new --output-certificate flag. It writes the certificate as base64 by default (given --b64 is true by default), but then if you try to use that same file to verify the blob later, it fails, as verify does not decode the string.

This makes --certificate works with both a base64'ed file and a regular one.

Not sure if this makes sense and if its the best impl, just throwing the idea around...

before

$ COSIGN_EXPERIMENTAL=1 go run ./cmd/cosign/ verify-blob --key key.pem --signature README.md.sig README.md
Error: verifying blob [README.md]: loading public key: pem to public key: PEM decoding failed
main.go:46: error during command execution: verifying blob [README.md]: loading public key: pem to public key: PEM decoding failed
exit status 1

now

$ ❯ COSIGN_EXPERIMENTAL=1 go run ./cmd/cosign/ verify-blob --cert key.pem --signature README.md.sig README.md
Certificate is trusted by Fulcio Root CA
Email: [carlos@becker.software]
Issuer:  https://github.com/login/oauth
Verified OK
tlog entry verified with uuid: "cca034dcf30323b30d8d131e06458c02187dab25ee1fa481914595777d027177" index: 877676

Ticket Link

n/a

Release Note

Allows base64 files as --cert in verify-blob