list models

Provide a command to list models. This will use credentials/urls of the defined provider connections.

```
$ kdn model list
PROVIDER     MODEL
myanthropic  opus-4-5
myanthropic  opus-4-6
myvertexai   gemini
...

$ kdn model list --provider myanthropic
PROVIDER     MODEL
myanthropic  opus-4-5
myanthropic  opus-4-6
```