No HA for RGW service

The documentation covers how to enable RGW module of MicroCeph.

https://canonical-microceph.readthedocs-hosted.com/en/squid-stable/how-to/enable-service-instances/#example-enable-an-rgw-service
https://canonical-microceph.readthedocs-hosted.com/en/squid-stable/reference/commands/enable/#rgw

> ```
> --port int Service non-SSL port (default: 80) (default 80)
> --ssl-port int Service SSL port (default: 443) (default 443)
> --ssl-certificate string base64 encoded SSL certificate
> --ssl-private-key string base64 encoded SSL private key
> --target string Server hostname (default: this server)
> --wait Wait for rgw service to be up. (default true)
> ```

However, there is no feature or content about how to create an HA radosgw endpoint. From a client point of view, one server node failure leads to a complete outage of radosgw service. In reality, HA entry point is a must-have (and it's also required for mgr modules such as dashboard).