Extend DriveToCheck for a runtime-property holding the free space

Refers to #20

ATxCommon/Serializables/DriveToCheck.cs

-namespace ATxCommon.Serializables
+using System.Xml.Serialization;
+
+namespace ATxCommon.Serializables
 {
     /// <summary>
     /// Helper class for the nested SpaceMonitoring sections.
@@ -14,5 +16,11 @@
         /// Limit (in GB) of free space, lower values will trigger a notification.
         /// </summary>
         public long SpaceThreshold { get; set; }
+
+        /// <summary>
+        /// Free space of a drive in bytes, set to -1 if unknown or check resulted in an error.
+        /// </summary>
+        [XmlIgnore]
+        public long FreeSpace { get; set; } = -1;
     }
 }
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