--public-fqdn argument for az fleet get-credentials

### Preconditions

- [x] No need to upgrade Python SDK or the Python SDK is ready.

### Related command

`az fleet get-credentials --resource-group group_name --name fleet_name`

### Resource Provider

N/A

### Description of Feature or Work Requested

The [az aks get-credentials](https://learn.microsoft.com/en-us/cli/azure/aks?view=azure-cli-latest#az-aks-get-credentials) command has a `--public-fqdn` argument to "Get private cluster credential with server address to be public fqdn."

The [az fleet get-credentials](https://learn.microsoft.com/en-us/cli/azure/fleet?view=azure-cli-latest#az-fleet-get-credentials) command does not have the same argument for accessing a private hub cluster. Can this be added?

### Minimum API Version Required

N/A

### Swagger PR link / SDK link

N/A

### Request Example

_No response_

### Target Date

2026-07-25

### PM Contact

N/A

### Engineer Contact

N/A

### Additional context

_No response_