[C#] Fix nested structs and arrays in Object API (#5765)

* [C#] Fix nested structs and arrays in Object API

The adds support for nested structs and fixed size arrays in the C#
Object API codegen which previously generated invalid code that wouldn't
compile.

- Nested structs would originally generate syntax errors due to adding an
  additional `.` to separate fields.
- Fixed size arrays of nested structs would originally generate code for
  the first field in the top most struct, and would lead to a compiler
  error due to referencing undefined variables.

* [C#] fix nested structs and arrays of structs.

* fix nested structs + arrays

* add table support

* Cleanup code

Co-authored-by: mugisoba <mugisoba+github@icloud.com>

tests/MyGame/Example/Vec3.cs

@@ -59,6 +59,8 @@ public struct Vec3 : IFlatbufferObject
   }
   public static Offset<MyGame.Example.Vec3> Pack(FlatBufferBuilder builder, Vec3T _o) {
     if (_o == null) return default(Offset<MyGame.Example.Vec3>);
+    var _test3_a = _o.Test3.A;
+    var _test3_b = _o.Test3.B;
     return CreateVec3(
       builder,
       _o.X,
@@ -66,8 +68,8 @@ public struct Vec3 : IFlatbufferObject
       _o.Z,
       _o.Test1,
       _o.Test2,
-      _o.Test3.A,
-      _o.Test3.B);
+      _test3_a,
+      _test3_b);
   }
 };