cli: improve ux for setting up new wireguard keys

Could this be handled by `change?`, e.g.:

```
admin@infix:/config/keystore/> change asymmetric-key my-wg-key
Please select key format: (R)SA or (X)25519
Generated X25519 key pair
```

_Originally posted by @wkz in https://github.com/kernelkit/infix/pull/1379#discussion_r2754289762_
            