Microceph does not come with an orchestrator backend module

# Issue report

There is currently no orchestrator backend deployed/packaged in the microceph snap distribution

## What version of MicroCeph are you using ?

reef/stable: 18.2.4+snapc9f2b08f92

## What are the steps to reproduce this issue ?

1. Open the dashboard
2. Change to menu "Cluster / Physical Disks"
3. See the warning from the dashboard
"Orchestrator is not available
Orchestrator is unavailable: Module not found
Please consult the [documentation](https://docs.ceph.com/en/latest/mgr/orchestrator) on how to configure and enable the Orchestrator management functionality."
4. Read the manual and try to set the orchestrator backend with:
`ceph orch set backend cephadm` or `ceph orch set backend rook`
5. List the modules with `ceph mgr module ls`

## What happens (observed behaviour) ?

For both modules `cephadm` and `rook` you get the error `Error EINVAL: Module 'cephadm' not found` or `Error EINVAL: Module 'rook' not found`.

The list of manager modules reveals that there are no cephadm or rook modules present.

```shell
MODULE
balancer on (always on)
crash on (always on)
devicehealth on (always on)
orchestrator on (always on)
pg_autoscaler on (always on)
progress on (always on)
rbd_support on (always on)
status on (always on)
telemetry on (always on)
volumes on (always on)
dashboard on
insights on
iostat on
nfs on
osd_perf_query on
osd_support on
prometheus on
restful on
alerts -
influx -
localpool -
mirroring -
selftest -
snap_schedule -
stats -
telegraf -
test_orchestrator -
zabbix -
```
## What were you expecting to happen ?

There is at least one suitable orchestrator backend coming with the microceph distribution in order to use the full potential of the dashboard.

Start by reproducing the warning in the dashboard's Cluster / Physical Disks view, then inspect the `ceph orch set backend` and `ceph mgr module ls` entry points for the MicroCeph snap. Done means a suitable orchestrator backend is packaged and deployable, and the dashboard no longer reports that the orchestrator module is unavailable.

Written by the indexing model from the issue text.