Refactor CallbackService class output in cpp_generator

[mattygiedt]

Moves the CallbackService class 'up' into the main gRPC Service class

Currently, if the --grpc-callback-api flag is passed to flatc, the $SERVICE$::CallbackService class is defined outside the main $SERVICE$ class. gcc (maybe others?) does not like the generated .grpc.fb.h file.

This PR moves the callback class 'up' into the main $SERVICE$ class.

[mattygiedt]

I actually think there's more to do here in order to use the various grpc Reactors. Trying to use a simple ServerUrnaryReactor I get the following segmentation fault:

gRPC version: 1.59.5

(gdb) bt
#0  0x000000000060535e in grpc::internal::CallbackUnaryHandler<flatbuffers::grpc::Message<CommandRequest>, flatbuffers::grpc::Message<CommandResponse> >::Deserialize (this=0x1b262d0, call=0x1bfe070, req=0x1bf3400, 
    status=0x1b5c0a0, handler_data=0x0) at /usr/local/include/grpcpp/impl/server_callback_handlers.h:94
#1  0x00000000009512cc in grpc::Server::SyncRequestThreadManager::DoWork(void*, bool, bool) ()
#2  0x00000000009561f0 in grpc::ThreadManager::MainWorkLoop() ()
#3  0x000000000095638c in grpc::ThreadManager::WorkerThread::Run() ()
#4  0x0000000000e71ac3 in grpc_core::(anonymous namespace)::ThreadInternalsPosix::ThreadInternalsPosix(char const*, void (*)(void*), void*, bool*, grpc_core::Thread::Options const&)::{lambda(void*)#1}::_FUN(void*) ()

Unclear on next steps, and / or if the builder_.SetContextAllocator interface will give us what we need.