provider creation

Introduce a new command `kdn provider create` and a system to register "provider services". The pattern should be similar to the secret service.

```
kdn provider create my-anthropic --type anthropic --token my-secret-token

kdn provider create my-vertexai --type vertexai --project my-project --region my-region --credentials ~/.config/gcloud/application_default_credentials.jso
```
