Add support for completion on private network for endpoint creation

### Description

At the moment there is no autocompletion performed on this field

```
scw inference endpoint create e3080ab0-4d2f-482f-a2f2-d8eb89e4fb24 endpoint.is-public=false endpoint.private-network.private-network-id= 
```

### How I imagine scw could expose this functionality

When I do an autocomplete here, I got a set of existing private network.

### References
