[FR] `az network bastion ssh --auth-type ssh-key` Ability to point to Key Vault secret

### Describe the bug

Would be great if `az network bastion ssh --auth-type ssh-key` could be pointed to a Key Vault secret containing the SSH private key, in addition to current ability to point to a local file.

How I'd like it to work:

```powershell
az network bastion ssh --subscription '' `
--resource-group '' `
--name '' `
--target-resource-id '/subscriptions//resourceGroups//providers/Microsoft.Compute/virtualMachines/' `
--auth-type 'ssh-key' --username '' --ssh-key 'https://.vault.azure.net/secrets/'
```

### Related command

```bash
az network bastion ssh --auth-type ssh-key
```

### Errors

```
Warning: Identity file https://.vault.azure.net/secrets/ not accessible: No such file or directory.
```

### Issue script & Debug output

None

### Expected behavior

Point to Key Vault secret using Azure CLI authenticated context should also work.

### Environment Summary

Not relevant.

### Additional context

_No response_