Support optional header in STFS, fix endianness in Descriptor (#80)

* Fix endianness in STFS Descriptor

* Support optional header for installer packages

* Fix field types

* Fix syntax

* Fix build

SabreTools.Data.Models/STFS/InstallerCacheHeader.cs

@@ -0,0 +1,41 @@
+using SabreTools.Numerics;
+
+namespace SabreTools.Data.Models.STFS
+{
+    /// <summary>
+    /// STFS Volume Descriptor, for System or Title Cache Installer STFS packages
+    /// </summary>
+    public class InstallerCacheHeader : InstallerHeader
+    {
+        /// <summary>
+        /// Resume state enum
+        /// See Enums.ResumeState
+        /// </summary>
+        /// <remarks>If present, 4 bytes</remarks>
+        public uint ResumeState { get; set; }
+
+        /// <summary>
+        /// Current file index
+        /// </summary>
+        /// <remarks>Big-endian</remarks>
+        public ulong CurrentFileIndex { get; set; }
+
+        /// <summary>
+        /// Number of bytes processed
+        /// </summary>
+        /// <remarks>Big-endian</remarks>
+        public ulong BytesProcessed { get; set; }
+
+        /// <summary>
+        /// Datetime for last modified
+        /// </summary>
+        /// <remarks>Microsoft FILETIME, Big-endian, 8 bytes</remarks>
+        public long LastModifiedDateTime { get; set; }
+
+        /// <summary>
+        /// Cache resume data
+        /// </summary>
+        /// <remarks>5584 bytes</remarks>
+        public byte[] ResumeData { get; set; } = new byte[5584];
+    }
+}