Update protobuf

[sagikazarmark]

This PR updates protobuf (with the relevant generators) and the related generated (and version controlled) code.

The code was generated using the following Nix shell setup:

{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/432fc2d9a67f92e05438dff5fdc2b39d33f77997.tar.gz") {} }:

pkgs.mkShell {
  buildInputs = [
    pkgs.git
    pkgs.go_1_16
    pkgs.protobuf
    pkgs.protoc-gen-go
    pkgs.protoc-gen-go-grpc
  ];
}

Related to #1109

[ChrisHines]

I'm not a gRPC expert, so I can't really validate all of the changes here. I do find it fascinating that go mod tidy doesn't make any changes to go.sum in this case, but that reproduces for me locally. Looks good as far as I can tell, though.

[sagikazarmark]

Most of the changes are in the code generator, but the new proto library should be backwards compatible with the old one. The rest of the changes only affect tests, not the library itself.

I guess the reason for no go.sum changes is that there are no observed changes in the list of dependencies: the new proto lib is a dependency of the old one (or vice versa?) and probably something else relies on the old library as well, so it doesn't disappear from go.sum. But I agree, the odds for this to happen are pretty low.