rgw/sfs: Implement missing functionality of get-user-info AdminOps API to get at least stats about number of objects

[votdev]

It would be great to implement the missing functionality of the get-user-info Admin Ops API endpoint in that way that the already existing stats object contains at least the number of objects (see num_objects) of the user in all buckets. This way we could add a dashboard widget in the UI. I think the number of objects in all buckets should be trivial to fetch from the internal database.

Right now the response data looks like that:

{
    "tenant": "",
    "user_id": "testid",
    "display_name": "M. Tester",
    ...
    "stats": {
        "size": 0,
        "size_actual": 0,
        "size_utilized": 0,
        "size_kb": 0,
        "size_kb_actual": 0,
        "size_kb_utilized": 0,
        "num_objects": 0       <========= Should be more trivial to populate than the other props
    }
}

[jecluis]

I suspect this is a duplicate of #82, only with a different endpoint?