Fixes#4133
This is pretty limited resource, as it covers only system mounts, but
this is all we need for KubeSpan for now. More complete solution should
probably involve COSIfying whole mount subsystem.
Example:
```
$ talosctl -n 172.20.0.2 get mounts
NODE NAMESPACE TYPE ID VERSION SOURCE TARGET FILESYSTEM TYPE
172.20.0.2 runtime MountStatus EPHEMERAL 1 /dev/vda6 /var xfs
172.20.0.2 runtime MountStatus STATE 1 /dev/vda5 /system/state xfs
```
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
