When initializing with `step ca init` there should be a prompt to asking if SSH CA should be enabled.

### What would you like to be added

If the `--ssh` flag is not passed in when using `step ca init` then the cli should prompt asking whether SSH CA should be enabled.

### Why this is needed

It's easy to initialize your CA PKI w/out knowing about the `--ssh` flag, and then the only recourse to go back and generate the SSH PKI resources is to regenerate the entire PKI/config.

