[C++] Rare bad buffer content alignment if sizeof(T) != alignof(T) (#7520)

* [C++] Add a failing unit test for #7516 (Rare bad buffer content alignment if sizeof(T) != alignof(T))

* [C++] Fix final buffer alignment when using an array of structs
* A struct can have an arbitrary size and therefore sizeof(struct) == alignof(struct)
  does not hold anymore as for value primitives.
* This patch fixes this by introducing alignment parameters to various
  CreateVector*/StartVector calls.
* Closes #7516

tests/alignment_test.h

@@ -0,0 +1,12 @@
+#ifndef TESTS_ALIGNMENT_TEST_H
+#define TESTS_ALIGNMENT_TEST_H
+
+namespace flatbuffers {
+namespace tests {
+
+void AlignmentTest();
+
+}  // namespace tests
+}  // namespace flatbuffers
+
+#endif