Support attribute "cpp_ptr_type" on table elements marked as "hash" (#4643)

* added intended use-cases to monster_test.fbs

* added check for `cpp_ptr_type` on hashed fields
added default value 'naked' to `cpp_ptr_type` on hashed fields

* added C++ generation of cpp_type vectors
removed ctor call for vector fields
added condition !vector for cpp_type check
added Pack() and UnPack() code generation for vector of hashes
added generation of correct resolve/rehash for cpp_type elements

* added attribute 'cpp_ptr_type_get' to hold accessor for pointer types possible where '.get()' does not work
use case: cpp_ptr_type:"std::weak_ptr", cpp_ptr_type_get:".lock().get()"

* run flatc to re-generate headers

* added bool param is_ctor to GetDefaultScalarValue() to differentiate between usage places

* modified monster_test.fbs to remove usage of shared_ptr/weak_ptr
reason: STLport does not support std::shared_ptr and std::weak_ptr

* run flatc again to re-generate headers

* fixed symbol unique_ptr not in namespace std when building with STLport

tests/test.cpp

@@ -20,6 +20,15 @@
 #include "flatbuffers/registry.h"
 #include "flatbuffers/util.h"
 
+// clang-format off
+#ifdef FLATBUFFERS_CPP98_STL
+  #include "flatbuffers/stl_emulation.h"
+  namespace std {
+    using flatbuffers::unique_ptr;
+  }
+#endif
+// clang-format on
+
 #include "monster_test_generated.h"
 #include "namespace_test/namespace_test1_generated.h"
 #include "namespace_test/namespace_test2_generated.h"