ci: Add formatting pre-submit check

[ianlewis]

Summary

Adds a formatting pre-submit check that runs make format and checks for file differences. The results of running make format are not included in this PR for readability. So the pre-submit will depend on #3655 and probably some more formatting before the check will pass.

Added a make shfmt formatting check for bash scripts.

autogen (for adding license headers) is now vendored in the third_party directory for convenience sake.

markdown-toc and autogen are now run by make format. The formatting pre-submit now runs these as well so they don't have separate pre-submit jobs. We will need to adjust the required checks on branch protection to remove "markdown-toc" and "autogen" jobs, and also add the "formatting" job.

CONTRIBUTING.md was updated to include some more information about coding style and conventions.

Testing Process

1. Run make format
2. Verify the formatting results

Checklist

• Review the contributing guidelines
• Add a reference to related issues in the PR description.
• Update documentation if applicable.
• Add unit tests if applicable.
• Add changes to the CHANGELOG if applicable.

[ianlewis]

Branch protections updated.