Add yapf to formatter

[oferb]

//tools:formatter currently relies on yapf being available on the user's system.
We should:

1. Make an http_archive that will make bazel download it
2. Add it to a BUILD file as a binary target, i.e py_binary
3. Add it to runtime_deps of //tools:formatter, so that it's available for the tool to use, and make the tool call it.

It's similar to how buildifier and clang-format work with the formatter.

Yapf: https://github.com/google/yapf/releases