[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_v1.json

@@ -0,0 +1,26 @@
+{
+  "a": 42,
+  "b": true,
+  "c_type": "TableB",
+  "c": {
+    "a": 15
+  },
+  "d": "King",
+  "e": {
+    "a": 3.1452,
+    "b": 325
+  },
+  "f":{
+    "a": 16,
+    "b": 243.980943
+  },
+  "g": [ 7, 8, 9],
+  "h": [
+    {
+      "a": 212
+    },
+    {
+      "a": 459
+    }
+  ]
+}