firebase-ios-sdk

Port FSTFirestoreClient to C++ 2/2 (#3657)

* firestore_client + FSTFirestoreClient becomes delegate.

* fix comment

* addressing comment

* use new instead make_shared

* reformat

* Update references to firestore client

* Addressing comments#2

* addressing comments #3

* add std::move

* remove deleted header.

* fix cmake build error.