[C++] Adds basic schema evolution tests (#5611)

* Added basic schema evolution tests

* Add BUILD targets for evolution tests. Added to test/generate_code scripts

* Use vector.front() instead of vector.data()

* Added --scoped-enums option for evolution test

tests/evolution_test/evolution_v2.json

@@ -0,0 +1,34 @@
+{
+  "b": false,
+  "c_type": "TableC",
+  "c": {
+    "a": 984.2494
+  },
+  "d": "Bishop",
+  "e": {
+    "a": 3.1452,
+    "b": 435,
+    "c": "yummy yummy fig bar bar"
+  },
+  "ff":{
+    "a": 35,
+    "b": 243.980943
+  },
+  "g": [ 7, 8, 10],
+  "h": [
+    {
+      "a": 212
+    },
+    {
+      "a": 459
+    },
+    {
+      "a": 333
+    }
+  ],
+  "i": 4321,
+  "j": {
+    "a": 9874.342,
+    "b": "more please"
+  }
+}