feat(xo-server/rest-api/dashboard): add S3 backup repositories information

[MathieuRA]

Description

XO metadata backups and Pool metadata backups are intentionally not counted, they do not have a size property at the moment

{
 ...,
 backupRepositories: {
    s3: {
        size: {
          backups: number
        }
    },
    other: {
       size: {
          available: number,
          backups: number,
          other: number,
          total: number,
          used: number
      }
    }
  } | undefined
}

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft