[planning-cpp] New header file providing the "optional" construct (otherwise only available in c++17)
[planning-cpp] New header file providing the "optional" construct (otherwise only available in c++17)

fire-rs-laas/fire-rs-saop,fire-rs-laas/fire-rs-saop,fire-rs-laas/fire-rs-saop,fire-rs-laas/fire-rs-saop