[Python] Build and publish grpcio-observability wheels for macos

Is your feature request related to a problem? Please describe.

Developing python applications that depend on grpcio-observability on macosx is challenging because there are no published .whl files targeting the OS on PyPI. It can be challenging and wasteful to setup an environment that will build the package from source on macos, and it can get messy to configure a cross-platform app to use bdist for linux but sdist for all other OSes.

Describe the solution you'd like

• Remove the explicit skip for grpcio-observability macos artifacts in tools/run_tests/artifacts/build_artifact_python.sh and make sure it runs the same checks as grpcio and grpcio-tools
• Update the PyPI docs and packaging metadata to include macos

https://github.com/grpc/grpc/issues/41611 addressed issues building grpc-observability for macos, so this should be unblocked.

Describe alternatives you've considered

I think the only alternative is to do nothing and build from source

Additional context

• The repository already has macOS Python artifact jobs that build universal2 wheels with ARCHFLAGS="-arch arm64 -arch x86_64"
• Relates to https://github.com/grpc/grpc/issues/38757