mgr/dashboard: introduce dashboard setting to resolve rgw hostname

mgr/dashboard: introduce dashboard setting to resolve rgw hostname

drops using the `rgw_dns_name` to resolve the rgw hosts in dashboard and
introduces a command `ceph dashboard set-rgw-hostname <daemon>
<hostname>` to set custom dns names for each gateways. Once set
dashboard will pick up that address for the selected gateway.

And a config can be unset by

`ceph dashboard unset-rgw-hostname <daemon>`

Fixes: https://tracker.ceph.com/issues/70744
Signed-off-by: Nizamudeen A <nia@redhat.com>