feat: VI-13156 Adds Python protoc.sh support

[richardlarocque]

VI-13156

Adds support for building Python protobuf client stubs. Adds pinned
versions for the relevant tools in versions.yaml.

This Python build script eschews fancier systems like poetry or uv
in favor of managing its own virtualenv. It maintains a virtualenv
under a .venv-codegen directory under the repo's root and installs the
necessary tools (grpcio, betterproto) there.

Also adds some placeholders for Python package building and publishing.

[getoutreach-ci-2]

Link to code coverage report (posted by coverbot 🤖)

[richardlarocque]

The RFC has more context and discussion around this PR.

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I'll acknowledge up front that this unlikely to be the long-term solution for protobuf generation. In the RFC discussion, we talked about moving some of this tooling more towards the repo and away from protoc.sh. There would be some merit in that approach.

The current approach is driven by the concern that this has to work. This protoc.sh script runs on make gogenerate, which runs with stencil, and it's really bad to break re-stencil-ing workflows. Therefore this approach aims to be as boring as possible and limit dependencies on external tools and developer laptop configuration.

The version presented in this PR should work so long as you have a Python on your path that is less than 10 years old + a network connection to PyPI. We dodge dependencies on poetry, AWS CodeArtifact, and other such things that would come with a more "repo-local" approach.

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Also, one more thing:

This code is flagged so it runs only in repos that declare a desire to generate Python gRPC stubs.

As I write this, the code to support such declarations is in PR. No such repos exist. We'll be opting them in one at a time over the next few weeks, based on the needs of the AI platform.

We'll get to try this out one repo at a time. It should be safe in that respect. Even if this code is somehow broken, we won't be breaking everyone all at once.