CI(clang-format): Post fixes as code suggestions if possible when running on a PR

[echoix]

This is the continuation of #1035, and will be adapted back to OSGeo/grass#3284 as a final solution.

After a hundred iterations or so, I've got a solution that I'm satisfied with. This allows linter formatting in a PR from a forked pull request to do so without any special secret access, and also be able to add PR review comments, that need pull-request: write permissions. These permissions cannot be granted for PRs coming from forks. Since all the PRs come from forks, we could never use any write permissions from our forks. It also avoids using pull_request_target with a checkout of the external code, which is something that should never be done.

Instead, it runs another workflow triggered by the completion of another workflow, through the workflow_run trigger. That second workflow takes 3 seconds, max 10 including startup. It won't appear as a check for a PR, but still manages to use the event's payload information available to apply to the PR. The fixes that need to be added as code suggestion comments are fed through the upload of a diff artifact. It is then easily downloaded in the second workflow.

Note that PR review comments can only be added to lines in the diff context, that is 3 lines above or below the changed lines of a PR, just like the web interface: the API isn't different. The code suggestion comments are added on a best-effort basis to facilitate contributors to finish a PR. So only upgrading the clang-format version in a PR won't be able to post suggestions on all changes in the repo, since the affected lines aren't changed in that PR.

I designed this solution to be able to accept and post code suggestions for other tools in the near future. Other workflows could upload diffs from multiple tools in the same artifact, and only the tool name and the file name needs to be handled by conditional expressions (if: ...) or a bash loop (careful to not inject the file names from the artifact, as it is untrusted inputs; prefer whitelisting the expected file names to match to). For now, it is implemented for clang-format.

This time, I created a new organization, so I could develop with the forked-repo situation from the start. Once this PR is merged, the "callback" workflow that post code suggestions will be enabled, and will work right away.