Enable building of gRPC Python with Bazel

gRPC Python currently has a constellation of scripts written to build the project, but it has a lot of limitations in terms of speed and maintainability. Bazel is the open-sourced variant of Google's internal system, Blaze, which is an ideal replacement for building such projects in a fast and declarative fashion. But Bazel in itself is still in active development, especially in terms of Python (amongst a few others).

This project aims to fill this gap and attempt to build gRPC Python with Bazel. Although filed under gRPC, this project also targets contribution to Bazel for Python.