gRPC callbackService support added (#8666)

* grpc callbackService support added

Signed-off-by: shankeleven <shashanksati11@gmail.com>

* tests: regenerate C++ gRPC golden with --grpc-callback-api (CallbackService & async_ reactor APIs); update formatting and method placement

---------

Signed-off-by: shankeleven <shashanksati11@gmail.com>
Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

grpc/tests/grpctest_callback_compile.cpp

@@ -0,0 +1,22 @@
+#include "monster_test.grpc.fb.h"
+
+// This test only verifies that the generated CallbackService compiles when the
+// callback API is available. It does not run any RPCs.
+
+#if defined(FLATBUFFERS_GENERATED_GRPC_CALLBACK_API) && \
+    defined(GRPC_CALLBACK_API_NONEXPERIMENTAL)
+class CallbackServiceImpl
+    : public MyGame::Example::MonsterStorage::CallbackService {
+ public:
+  // For brevity we don't override methods; user code will provide reactors.
+};
+#endif
+
+int main() {
+#if defined(FLATBUFFERS_GENERATED_GRPC_CALLBACK_API) && \
+    defined(GRPC_CALLBACK_API_NONEXPERIMENTAL)
+  CallbackServiceImpl svc;
+  (void)svc;  // suppress unused
+#endif
+  return 0;
+}