tunnelbroker: Generate gRPC/protobuf code with CMake

Summary:
Solves https://linear.app/comm/issue/ENG-777/generate-grpcproto-code-as-cmake-target

Also sets MAKEFLAGS for parallelism during build, which can be overridden with Docker build argument.

Test Plan: Run the tunnelbroker service launch and test scripts

Reviewers: ashoat, karol-bisztyga, geekbrother, varun

Reviewed By: ashoat, karol-bisztyga

Subscribers: palys-swm, Adrian, atul, benschac

Differential Revision: https://phabricator.ashoat.com/D3236