[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/Gadget.kt

@@ -0,0 +1,13 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+@Suppress("unused")
+@ExperimentalUnsignedTypes
+class Gadget private constructor() {
+    companion object {
+        const val NONE: UByte = 0u
+        const val FallingTub: UByte = 1u
+        const val HandFan: UByte = 2u
+        val names : Array<String> = arrayOf("NONE", "FallingTub", "HandFan")
+        fun name(e: Int) : String = names[e]
+    }
+}