Please add support for `ssh-add -c` to `step ssh login`

[dionorgua]

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Issue details

It would be cool to have a way to add just generated SSH client cert to agent with -c option to require confirmation on every use.

Something like:

step ssh login --confirm

I think it also makes sense to add same thing to step ssh proxycommand to make it automatic

Why is this needed?

I know that it's less important for very short-lived certs, but I think it still makes sense to require user confirmation for every certificate use