[C++] Add option to not generate direct copy methods. (#6166)

* Add option to not generate direct copy methods.

The direct copy methods generated by flatc utilize std::vector which isn't allowed on some embedded systems. Permit users of the compiler to not generate these methods so they don't have to be stubbed out.

* Update docs for no-cpp-direct-copy option.

src/flatc.cpp

@@ -282,6 +282,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.cpp_object_api_string_type = argv[argi];
       } else if (arg == "--cpp-str-flex-ctor") {
         opts.cpp_object_api_string_flexible_constructor = true;
+      } else if (arg == "--no-cpp-direct-copy") {
+        opts.cpp_direct_copy = false;
       } else if (arg == "--gen-nullable") {
         opts.gen_nullable = true;
       } else if (arg == "--java-checkerframework") {

src/idl_gen_cpp.cpp

@@ -2398,7 +2398,7 @@ class CppGenerator : public BaseGenerator {
     }
 
     // Generate a CreateXDirect function with vector types as parameters
-    if (has_string_or_vector_fields) {
+    if (opts_.cpp_direct_copy && has_string_or_vector_fields) {
       code_ +=
           "inline flatbuffers::Offset<{{STRUCT_NAME}}> "
           "Create{{STRUCT_NAME}}Direct(";