The `import/copybara` status check should be required for merging AND expose its output

[sudoforge]

Currently, the import/copybara status check is not a required status check, and its output is hidden, making it difficult/impossible for external contributors to debug any failure with it.

I propose that the status check should:

• Be selected as a "required status check" in the Branch Protection settings

• Expose its output such that external contributors can see error messages and derive why it is failing.

[hsudhof]

The opacity of the check is sadly an artifact of our development model that also makes it impossible to share meaningful error messages on GitHub.
Our source of truth is a non-GitHub git instance and we use copybara with a GitHub app for the export - i.e. we do not merge on GitHub and we cannot use branch protection because it doesn't support apps well.

Thank you for the feedback, we hope to be able to do some of these things in the future :)

[sudoforge]

Totally understand that there's more at play here -- and that merging doesn't happen on Github. Nonetheless, setting it as a required status check (just like the CLA verification) would provide good visual feedback to external contributors.

I created this simply to track the effort in a public manner, with the understanding that it would be a longer term project. Glad to see some alignment there!