New: Add backup size information

Closes #4830

(cherry picked from commit 78aeda1a2cc217c367c5c3c6fd281c101b28413c)

src/Radarr.Api.V3/System/Backup/BackupController.cs

@@ -42,6 +42,7 @@ namespace Radarr.Api.V3.System.Backup
                 Name = b.Name,
                 Path = $"/backup/{b.Type.ToString().ToLower()}/{b.Name}",
                 Type = b.Type,
+                Size = b.Size,
                 Time = b.Time
             })
                                        .OrderByDescending(b => b.Time)

src/Radarr.Api.V3/System/Backup/BackupResource.cs

@@ -9,6 +9,7 @@ namespace Radarr.Api.V3.System.Backup
         public string Name { get; set; }
         public string Path { get; set; }
         public BackupType Type { get; set; }
+        public long Size { get; set; }
         public DateTime Time { get; set; }
     }
 }