[Feature] Allow multiple SSH keys for ssh target

[alistarle]

As a warpgate administrator, I would like to specify a SSH key in a target configuration instead of using the default warpgate-keys (rsa or ecdsa) in order to use warpgate to connect to different environment and different machine who have different keys.

Indeed, only one private ssh key can be set in warpgate and it should be present on all ssh target.

If it looks okay to you, we can develop this feature.