Set C# Struct/Table visibility to public (#5381) (#5416)

net/FlatBuffers/Struct.cs

@@ -19,7 +19,7 @@ namespace FlatBuffers
     /// <summary>
     /// All structs in the generated code derive from this class, and add their own accessors.
     /// </summary>
-    internal struct Struct
+    public struct Struct
     {
         public int bb_pos { get; private set; }
         public ByteBuffer bb { get; private set; }