chore: add generation of pb2 files via owlbot/bazel

[parthea]

This PR is built on top of #122 and #123. This PR requires changes to build rules in googleapis/googleapis. I'll mark this PR as ready for review once the changes to the build rules are live.

I tested the changes locally using the commands below and the results are in #125.

In a clone of googleapis/googleapis, run

gbazelisk build //google/iam/v1/logging:audit-data-py
gbazelisk build //google/iam/v1:iam-v1-py

The in a clone of this repo, run

docker run --rm --user $(id -u):$(id -g)   -v $(pwd):/repo   -v <path to your bazel-bin>:/bazel-bin   gcr.io/cloud-devrel-public-resources/owlbot-cli:latest copy-bazel-bin   --source-dir /bazel-bin --dest /repo

and

docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo gcr.io/cloud-devrel-public-resources/owlbot-python:latest

[parthea]

Wait for cl/710118578