Add -k8s flag to generate-key-pair

[priyawadhwa]

This command will:

1. generate the key-pair in memory
2. prompt the user for a password
3. store cosign.pub, cosign.key and cosign.password as a Kubernetes secret
4. store cosign.pub to disk

This will be useful for chains!

Usage:

cosign generate-key-pair -k8s default/cosign-secret

Outputs a secret with the following data fields:

• cosign.key (private key)
• cosign.pub (public key)
• cosign.password (password)

[dlorenc]

This looks great! One nit about the way the secret is stored/accessed. Do you think it would make sense to also support passing this as a ref to cosign sign -key k8s://<namespace>/<secret>...? Same for verify. Those can be todos or issues though.

[priyawadhwa]

Do you think it would make sense to also support passing this as a ref to cosign sign -key k8s:///...? Same for verify.

I could see people using this if they're running cosign in k8s as well -- opened up #347 to track it.