Improve XSite Rest Endpoint

Endpoint: cache-managers/<name>/x-site/backups/

The return value is incomplete. With "mixed" status it should include the caches where the "site2" is online and offline (and mixed too).

{
  "site2": {
    "status": "mixed",
    "online": [],
    "offline": []
  }
}

Endpoint caches/<name>/x-site/backups/

As the endpoint above, it should return the node name where the site is online and offline, instead of a string where users have to parse it.

{
  "site2": "mixed, offline on nodes: [pedro-laptop-3-70]"
}