[UX] Do not require the client users of SSH fleets to provide SSH key for the proxy jump node

[peterschmidt85]

Currently, if I set up an SSH fleet that uses a proxy jump, any users that want to use such a fleet have to update their SSH config and specify the SSH key for the proxy jump node.

This is especially inconvenient for teams where users are not involved in the setup of SSH fleets and might not even know which SSH fleets are confirmed.

The ideal situation if the server would automatically configure this key on the client side.