Use bazel rule to compile protos during the initial generation

[summer-ji-eng]

Current: bazel generates a library without compiled protos, so it's expected that someone (synthtool, OwlBot, etc.) will call "npm run compile-protos" later in the process

Downside: so it's a post-processing step that is quite expensive and fragile

Improve: would be better if the protos were compiled during the initial generation step by bazel, to reduce the number of things that could break in post-processing

Discuss this feature in June 2021 sprint