Gazelle for C++ BUILD file generation

[udaya2899]

Gazelle generates BUILD files for Go so the author of Go targets doesn't have to write BUILD files on their own by hand.

Gazelle luckily has a generic implementation that can be extended using their extension interface to support other languages. rules_python implemented this extension to support Gazelle.

There have been many requests and interest from people at bazelbuild/bazel-gazelle#910 and I believe there is huge community interest and value for this feature.

Other resources:

• Talk about Gazelle
• One cpp implementation from a Google Open Source project - skia