[C++] Fix compile failure on Object API union construction for struct member (#6923)

* Add dedicated traits to Object API version of unions.

* Add suppression for unused parameters on unions of structs.

tests/union_vector/FallingTub.cs

@@ -0,0 +1,49 @@
+// <auto-generated>
+//  automatically generated by the FlatBuffers compiler, do not modify
+// </auto-generated>
+
+using global::System;
+using global::System.Collections.Generic;
+using global::FlatBuffers;
+
+public struct FallingTub : IFlatbufferObject
+{
+  private Struct __p;
+  public ByteBuffer ByteBuffer { get { return __p.bb; } }
+  public void __init(int _i, ByteBuffer _bb) { __p = new Struct(_i, _bb); }
+  public FallingTub __assign(int _i, ByteBuffer _bb) { __init(_i, _bb); return this; }
+
+  public int Weight { get { return __p.bb.GetInt(__p.bb_pos + 0); } }
+  public void MutateWeight(int weight) { __p.bb.PutInt(__p.bb_pos + 0, weight); }
+
+  public static Offset<FallingTub> CreateFallingTub(FlatBufferBuilder builder, int Weight) {
+    builder.Prep(4, 4);
+    builder.PutInt(Weight);
+    return new Offset<FallingTub>(builder.Offset);
+  }
+  public FallingTubT UnPack() {
+    var _o = new FallingTubT();
+    this.UnPackTo(_o);
+    return _o;
+  }
+  public void UnPackTo(FallingTubT _o) {
+    _o.Weight = this.Weight;
+  }
+  public static Offset<FallingTub> Pack(FlatBufferBuilder builder, FallingTubT _o) {
+    if (_o == null) return default(Offset<FallingTub>);
+    return CreateFallingTub(
+      builder,
+      _o.Weight);
+  }
+}
+
+public class FallingTubT
+{
+  [Newtonsoft.Json.JsonProperty("weight")]
+  public int Weight { get; set; }
+
+  public FallingTubT() {
+    this.Weight = 0;
+  }
+}
+