Support 'cosign sign' with private keys handled by SSH agent

[jas4711]

Hi. It seems 'cosign sign' supports on-disk private keys and PKCS#11 tokens, PIV cards, AWS KMS, GCP KMS, Azure Key Vault, Hashicorp Vault. But there is one fairly common way to access private-keys that is missing: through a SSH agent socket.

Am I missing anything, or is support for signing via a SSH agent socket missing from cosign?

If so, please consider this a request to add support for it.

There is a bunch of SSH agents providing access to private keys held on OpenPGP cards, TPMs, etc that would then be accessible for use by Sigstore cosign.

Thanks,
/Simon