firebase-ios-sdk

Fuzz Testing Firestore using CMake (#1593)

* Added `cc_binary` rule to allow executables.
* Added `libFuzzer` as an external project that Firestore depends on.
  * LibFuzzer is checked out from its SVN repository.
  * LibFuzzer is built using its `build.sh` script to produce the library `libFuzzer.a`.
  * Added a `FindLibFuzzer.cmake` script that looks for `libFuzzer.a`.
* Added a new cmake option `FUZZING` to enable fuzzing and adding the appropriate compiler flags.
  * Added `fuzzing_options.cmake` rules to check for compiler type and add the fuzzing flags accordingly.
* Added a `firebase_firestore_fuzzing_serializer` target that depends on `libFuzzer` and `firebase_firestore_remote` to fuzz test the serializer.