mgr/dashboard: fix missing schedule interval in rbd API

mgr/dashboard: fix missing schedule interval in rbd API

Fetching the rbd image schedule interval through the rbd_support module
schedule list command

GET /api/rbd will have the following field per image
```
"schedule_info": {
                    "image": "rbd/rbd_1",
                    "schedule_time": "2025-09-11 03:00:00",
                    "schedule_interval": [
                        {
                            "interval": "5d",
                            "start_time": null
                        },
                        {
                            "interval": "3h",
                            "start_time": null
                        }
                    ]
                },
```

Also fixes the UI where schedule interval was missing in the form and
also disable editing the schedule_interval.

Extended the same thing to the `GET /api/pool` endpoint.

Fixes: https://tracker.ceph.com/issues/72977
Signed-off-by: Nizamudeen A <nia@redhat.com>