Feature request: support adding keys to ssh-agent

[jasonkarns]

For users that use gpg keys for ssh auth, it would be amazing if this action also set up for forwarding of ssh-agent to gpg-agent such that we could use our gpg keys for SSH auth.

[crazy-max]

@jasonkarns LGTM, I think we would only need to add enable-ssh-support to gpg-agent conf and maybe specify keys in gnupg sshcontrol but we would need to handle keygrips somehow. Last we have to tell SSH how to access the gpg-agent. Maybe use SSH_AUTH_SOCK for that.

[jasonkarns]

I've realized later that this probably won't work for my primary use case...

My thought was to have a single gpg key for signing+auth which would be installed using this action. The signing key would be used to sign git commits, and the auth key would be used for authenticating with github for the push/pull.

However, there's a bit of a catch-22. This action must run after the checkout action, in order to support setting the gpg signing key. However, it would need to run before the checkout action if one wanted to provide the auth key to the checkout action via the ssh-key parameter.

So while I still think there may be some utility here, my personal interest has waned a bit. Unless this feature were to "double up" on whatever the checkout action's ssh-key param is doing (and thereby cut the catch-22 loop).