Migrate recent HLLib work to BOS

It's better suited for the model of BOS and not an update to HLLibSharp

BurnOutSharp.Models/SGA/Header6.cs

@@ -0,0 +1,14 @@
+namespace BurnOutSharp.Models.SGA
+{
+    /// <see href="https://github.com/RavuAlHemio/hllib/blob/master/HLLib/SGAFile.h"/>
+    public sealed class Header6 : Header
+    {
+        public string Name;
+
+        public uint HeaderLength;
+
+        public uint FileDataOffset;
+
+        public uint Dummy0;
+    }
+}