firebase-ios-sdk

Embed root certificates into the binary on Apple platforms as well (#4352)

Currently, on iOS Firestore is loading `roots.pem` file from the gRPC bundle. The mechanism has proved to be brittle, so instead we can use the same approach as the CMake build does -- parse the file at build time and embed its contents into the binary.

One downside of this approach is that now we would have to periodically update the generated files for embedding manually. Judging from history, `roots.pem` is updated quite often in gRPC repo, and can be expected to be modified on each release.

The command to regenerate the certificates is something like:
```sh
mkdir build # From the root of the repo
cd build
cmake .. && make -j 8 generate_grpc_root_certificates
```

TESTED=with Fireeats.