[nix] Add protoc-gen-grpc-web for gRPC web codegen

Summary: We need this to codegen JS files from .proto files. Nix has version 1.3.1, which is close to the latest on GitHub releases (1.4.2), which is where the official docs (first link) point you to.

Test Plan:
After making this change I exited and reentered my Nix environment and confirmed that the tool was installed based on the output of nix develop.

I then ran this command which I got from the "Basics tutorial" I found in the gRPC web docs:

protoc -I=shared/protos/ identity_client.proto --js_out=import_style=commonjs:.

It generated a identity_client_pb.js file that looked right.

Reviewers: jon, varun, atul, O1 Nix

Reviewed By: varun, O1 Nix

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7270