[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.fbs

@@ -0,0 +1,39 @@
+namespace Evolution.V1;
+
+table TableA {
+    a:float;
+    b:int;
+}
+
+table TableB {
+    a:int;
+}
+
+enum Enum : byte {
+    King,
+    Queen
+}
+
+union Union {
+    TableA,
+    TableB
+}
+
+struct Struct {
+    a:int;
+    b:double;
+}
+
+table Root {
+    a:int;
+    b:bool;
+    c:Union;
+    d:Enum;
+    e:TableA;
+    f:Struct;
+    g:[int];
+    h:[TableB];
+    i:int = 1234;
+}
+
+root_type Root;