[RemoteConfig]V2 - CPP API change for API review
[RemoteConfig]V2 - CPP API change for API review

go/new-rc-cpp-unity-design
go/new-rc-cpp-unity-api

PiperOrigin-RevId: 284864568

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk