Add multiple SSH key pairs

[Francesco Di Nucci]

Actually (at least up to 2023.1 Antelope) when launching a new instance through the dashboard it is only possible to add a single key pair. There is a [workaround](https://help.switch.ch/engines/faq/how-to-use-multiple-ssh-keys/) but only works through CLI.

It would be nice in the web UI to be able to select multiple key pairs for an instance (for example because there are multiple users that should access the same instance).