This adds a PRESUBMIT.py file for infra/config, which works with `git cl
presubmit` from depot_tools. The presubmit function checks that the
lucicfg generated output matches the changes to main.star and has no
validation errors.
Fixes the existing config file to make it conform to the presubmit
checks (e.g. formatting, docstrings).
Also adds a mention of depot_tools to CONTRIBUTING.md to make it clear
that depot_tools works on the boringssl repo.
Change-Id: I311cab911b80025db5c0d780f8959b58c033d81c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80627
Auto-Submit: Lily Chen <chlily@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
