Ability to get RGW public IPs

# Issue report

This is an enhancement to retrieve RGW public IPs.

In the current scenario, the public IPs of mon's are stored in the db table config as host.mon., value.
Whenever microceph requires public IPs to set in ceph configs, it uses the above table contents.

And the charm microceph retrieves the public IPs of monitors by parsing the ceph config.
(I believe this can be better handled than reading ceph config)

However there is no such mechanism to retrieve public IPs of RGW either via CLI or Daemon API.

Use case:
Sunbeam uses traefik for Ingress to RGW nodes and the traefik configuration requires RGW public IPs to set as backend.