Adds dependencies on the gRPC & protobuf libraries, and implements Nix
code to generate the C++ sources from the included proto definitions.
This is theoretically supported via CMake, but practically doesn't
work and I don't care to debug why.
Doing it like this lets us instead add a CMake library target for our
proto definitions based on the sources generated by Nix.
Pros:
* no need to deal with the gRPC CMake mess
* it works!
Cons: * iteration requires nix-shell restart
Change-Id: Ie1fe9807fc96c49cb8f7161ba59d093456062b15
Reviewed-on: https://cl.tvl.fyi/c/depot/+/927
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
0064e55486