CI: add github action to block merging large files

[camdencheek]

This adds a github action which checks that the PR doesn't add any files greater than 1MB. We've had a lot of large binaries or test files be committed recently, so this should help block those from making it to main, since once they're in main, they're in the history permanently.

Test plan

See example failure here.

[camdencheek]

Converted to draft because the skip logic doesn't seem to work

[eseliger]

bash isn't my strength but if it works great!

[jhchabran]

It's great!

Do you mind adding a small snippet that comments on the PR as well? It's a rather low hanging fruit:

jobs:
  comment:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@v6
        with:
          script: |
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '👋 Thanks for reporting!'
            })

[camdencheek]

@jhchabran, thanks for the suggestion! Will add.

I converted this back to draft because I couldn't get the "override this check" working, and I think we want a "break glass" option. Do you have recommendations on how to handle that?

[jhchabran]

@camdencheek there's AFAIK no easy way to do that, because we either allow specific actors to bypass all checks or we don't. I think given the context of thir particular check, it should be fairly rare that someone has to break glass. And if they did, they can reach us on the support channel.

So I'm in favour of adding it as is, and we could totally print out a message telling folks to do so in the GitHub action error, or if you want to take it further, it's not hard to post a comment from the action itself on the PR to really surface that. I'm fine with both.