Allow extra options to be passed to `ssh` calls.

[soupglasses]

Is your feature request related to a problem? Please describe.

Ssh configurations can be custom defined, when IdentitiesOnly enabled, ssh will no longer iterate over its ~/.ssh/ keys. Instead of requiring each user to configure their global ~/.ssh/config per install, i would propose an extra option, --ssh-option which would allow you to set arguments that are passed into each ssh call, similar to how nix has it with --option.

Describe the solution you'd like

This can help greatly by allowing a pathway to manually define an ssh key.

For example: --ssh-option '-i ~/.ssh/key'.

Describe alternatives you've considered

Manually writing/changing each machine's .ssh/config to make it work with nixos-anywhere. But this seems less than ideal and requires a lot of manual intervention unless ssh defaults are strictly followed by everyone.

Additional context

None