Feature #48314: mgr/dashboard: reverse proxy support
mgr/dashboard: enable different URL for users of browser to Grafana
To be able to cope with FQDNs and various other types of custom setups, where users of the Ceph Dashboard do not have access to Grafana through the bare hostname, a second configuration option shall be added to the dashboard. This enables the dashboard to serve that URL to users of a browser, whereas the current (first) URL shall be used for the connection from the backend to Grafana (to verify Dashboards exists, etc).
When only one URL is configured, the behavior of the dashboard should not change, meaning that this URL should be used for both, the backend and be served to the frontend.
This implementation is trivial, though enables a great configuration flexibility.