mgr/dashboard/backend: Enable get/set of cluster-wide OSD settings

mgr/dashboard/backend: Enable get/set of cluster-wide OSD settings

Add ability to list, set and unset cluster-wide OSD flags.

Flags can be listed and changed through the `/api/osd/flags` API
resource. By using a GET request, the list is retrieved. By using a PUT
request, the flags are updated (all at once). Flags not contained in the
data of the PUT are removed, additional once are added. Note that the
PUT requests require a JSON body with the data contained as value of the
'flags' key like so:

    {"flags": ["flag1", "flag2", ...]}

Fixes: http://tracker.ceph.com/issues/24056
Signed-off-by: Patrick Nawracay <pnawracay@suse.com>