[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.kt

@@ -0,0 +1,27 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import java.nio.*
+import kotlin.math.sign
+import com.google.flatbuffers.*
+
+@Suppress("unused")
+@ExperimentalUnsignedTypes
+class FallingTub : Struct() {
+
+    fun __init(_i: Int, _bb: ByteBuffer)  {
+        __reset(_i, _bb)
+    }
+    fun __assign(_i: Int, _bb: ByteBuffer) : FallingTub {
+        __init(_i, _bb)
+        return this
+    }
+    val weight : Int get() = bb.getInt(bb_pos + 0)
+    fun mutateWeight(weight: Int) : ByteBuffer = bb.putInt(bb_pos + 0, weight)
+    companion object {
+        fun createFallingTub(builder: FlatBufferBuilder, weight: Int) : Int {
+            builder.prep(4, 4)
+            builder.putInt(weight)
+            return builder.offset()
+        }
+    }
+}