CI: Add jobs to reveal changes in create-plugin output

[leventebalogh]

What is the problem?

Refactoring @grafana/create-plugin is not an easy task - it is becoming quite complex, and we don't really have automated test coverage to validate if the generated output remains the same (again, this is only important for refactors, where the functionality has to remain the same).

Solution idea

After brainstorming this with @jackw, we roughly have the following idea(s):

• We should introduce some new checks to the CI, which are not blocking (it's important, because anything else than refactoring should change the generated code)
• The new checks should somehow reveal the difference between the latest version of @grafana/create-plugin and the version in the PR. We would think a comment on the PR would be the best, but with folding the diff output in the comment (in case there is any).
• The best would be probably to add the check to the matrix steps, so we can "reuse" the existing matrix scenarios. This makes it bit harder however to display the diff of the different scenarios in a single comment in a way that it remains easy to digest.

[github-actions[bot]]

Hello! 👋 This repository uses Auto for releasing packages using PR labels.

✨ This PR can be merged. It will not be considered when calculating future versions of the npm packages and will not appear in the changelogs.

[CLAassistant]

All committers have signed the CLA.