request: add flatbuffers.rb

[ahundt]

Flatbuffers is also supported with grpc, it would be great if a homebrew script could be added for setting it up with grpc flatbuffers integration working

[ahundt]

note homebrew provides a flatbuffers installer:
https://github.com/Homebrew/homebrew-core/blob/master/Formula/flatbuffers.rb

But I'm not sure integration with grpc will work with the current version.