The change updates how clang-format is called in CI and locally, and adds prettier to the pre-commit hook. Proto files are now also formatted, while external files are excluded.
* Add a new YAML file (.pre-commit-config.yaml) to set up pre-commit
hook for clang-format
* The pre-commit hook is opt-in and needs to be installed in order to
run automatically
* Update CONTRIBUTING.md with instructions on how to set up and use the
clang-format linter
Automating the process of running clang-format before committing code
helps to save time by removing the need to fix formatting issues later.
This is a tooling improvement and doesn't change C++ code.
