New: Add config file setting for CGNAT authentication bypass

(cherry picked from commit 4c41a4f368046f73f82306bbd73bec992392938b)

Closes #3903

src/NzbDrone.Common/Extensions/IpAddressExtensions.cs

@@ -52,5 +52,11 @@ namespace NzbDrone.Common.Extensions
 
             return isLinkLocal || isClassA || isClassC || isClassB;
         }
+
+        public static bool IsCgnatIpAddress(this IPAddress ipAddress)
+        {
+            var bytes = ipAddress.GetAddressBytes();
+            return bytes.Length == 4 && bytes[0] == 100 && bytes[1] >= 64 && bytes[1] <= 127;
+        }
     }
 }