Describe the feature or problem you’d like to solve

gh ssh-key add adds keys with type "authentication key" only. There is no way to submit them as "signing key"s

Proposed solution

Add a second parameter in addition to the title parameter to specify the key type.

gh ssh-key list should ideally also be modified to show the key type

Additional context

This allows for signature verification with SSH keys. Currently the only solution that I could find was to upload the key in the UI to select the key type

Ref: https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification#ssh-commit-signature-verification